Remove soft deprecated literal warning (#4660)

Every use of `literal/1` generates a warning that directs people to use
`identifier/1` instead. According to the CHANGELOG, this is a soft
deprecation and it shouldn't generate so much noise.

Furthermore, this warning causes a great deal of noise for users of
libraries that support multiple versions of Ecto.

Author: sorentwo

lib/ecto/query/builder.ex

@@ -760,11 +760,6 @@ defmodule Ecto.Query.Builder do
         env -> env
       end
 
-    IO.warn(
-      "`literal/1` is deprecated. Please use `identifier/1` instead.",
-      Macro.Env.stacktrace(env)
-    )
-
     escape_fragment({:identifier, meta, [expr]}, params_acc, vars, env)
   end