test(protocols): test_mh_unit's _drop_tracebacklimit cleanup has been dead since #362 #1924

tests/protocols/internet/test_mh_unit.py:1667-1675 registers a cleanup that deletes sys.tracebacklimit because "the loud _missing_ ... sets this process-wide". Since #362, no pcapkit code sets sys.tracebacklimit: the BaseError docstring's versionchanged:: 1.5.0 says so, and a grep of pcapkit/ finds no assignment. The cleanup therefore does nothing useful. Because it deletes the attribute whenever it exists, it would also remove a value the test runner had set.

#1915 already corrects the comment but leaves the code.

Fix: once #1915 lands, drop the cleanup, plus import sys if nothing else in the test uses it.

Found in the #1103 round-4 sweep.

Blocked until #1915 merges, since that PR rewrites the same comment.