[math] Remove unused aggregate LinkDef headers

[kutsibalci]

What

Removes two aggregate LinkDef headers that are no longer part of any dictionary:

• math/smatrix/inc/LinkDefAll.h
• math/genvector/inc/Math/LinkDef_GenVectorAll.h

One commit per module, since the two are independently revertable.

Why

Both were added in 880fe6d (2014-01-30, "Fix the CMake build"), when each package built a
single combined dictionary. Each file exists only to pull its two real LinkDefs together:

// math/smatrix/inc/LinkDefAll.h
#include "LinkDefD32.h"
#include "LinkDef.h"

The build has since returned to two separate dictionaries per package, each with its own LinkDef:

package	dictionary	LINKDEF
Smatrix	G__Smatrix	LinkDef.h
Smatrix	G__Smatrix32 (MULTIDICT)	LinkDefD32.h
GenVector	G__GenVector	Math/LinkDef_GenVector.h
GenVector	G__GenVector32 (MULTIDICT)	Math/LinkDef_GenVector32.h

That leaves the aggregate headers unreferenced. Neither name appears in any CMakeLists.txt or
.cmake file in the repository, and no other LinkDef includes them. They are also not reachable
from outside the repository: ROOT_INSTALL_HEADERS excludes LinkDef headers from installation
(cmake/modules/RootMacros.cmake:1237, set(options REGEX "LinkDef" EXCLUDE)).

Verification

Configured a build with both files removed (Ninja, -DCMAKE_BUILD_TYPE=Release -Dtesting=ON -Droottest=OFF, GCC 13.3 on Ubuntu 24.04). Configuration completes with no errors, and the
generated build.ninja contains:

reference	occurrences
LinkDefAll	0
LinkDef_GenVectorAll	0
math/smatrix/inc/LinkDef.h	2
LinkDefD32.h	2
LinkDef_GenVector.h	2
LinkDef_GenVector32.h	2

All four dictionaries are still generated: G__Smatrix.cxx, G__Smatrix32.cxx,
G__GenVector.cxx, G__GenVector32.cxx.

Related

Same class of leftover as #23002 ([tmva] Remove unused LinkDef5.h), found by the same sweep.
The two are independent; this one can be taken or dropped on its own.

For completeness, the sweep flagged two further candidates that I am not proposing here:
test/guiviewerLinkDef.h, because guiviewer.cxx/guiviewer.h are still present and documented
in test/Readme.md, so the LinkDef is only as orphaned as the example itself; and the LinkDefs
under math/experimental/genvectorx/, because that package builds no dictionary at all today and
is only configured with sycl AND experimental_genvectorx. Both look like questions for the
owners rather than obvious removals — happy to open an issue on either if useful.

AI-assisted coding disclosure

This contribution was AI-assisted (Claude Code). The tool was used to build a reachability graph
over every LinkDef file in the repository (CMake references, #include edges between LinkDefs,
and the default LinkDef.h convention), and to draft this description.

The results were verified before opening the PR rather than taken on trust: both files were
confirmed unreferenced repository-wide, the 2014 commit that introduced them was read, the current
CMakeLists.txt dictionary definitions were checked by hand, the installation exclusion was
located in RootMacros.cmake, and the configuration check above was run against the branch with
both files already deleted. Two of the four candidates the sweep produced were set aside as
described above rather than bundled in.

I have reviewed and understood the change and take responsibility for it.

[kutsibalci]

One CI job is red — ubuntu22 imt=Off, CMAKE_BUILD_TYPE=Debug. I looked into it and believe it is unrelated to this PR, but flagging it rather than leaving it unexplained.

• The failing test is roottest-root-io-cpp11Containers-unorderedMap, from the test results artifact for that job. It exercises I/O of C++11 unordered containers, which does not touch Smatrix or GenVector dictionaries.
• 20 of the 21 jobs in the run passed, including every other platform (alma9, alma10, alma10 arm64, fedora43, fedora44, ubuntu2404, mac14, mac15 ARM64). A change that broke dictionary generation would not be confined to a single Debug configuration.
• There are no compiler-error annotations on the job; it failed after ~79 minutes at the test stage, not during configure.
• The same job passed on [tmva] Remove unused LinkDef5.h #23002, which is the same kind of removal.

Happy to rebase if you would like it re-run, or to dig further if you think it is genuinely related.

[github-actions]

Test Results

    23 files      23 suites   3d 16h 20m 3s ⏱️
 3 853 tests  3 852 ✅ 0 💤 1 ❌
79 408 runs  79 406 ✅ 1 💤 1 ❌

For more details on these failures, see this check.

Results for commit 5dbd790.