gcc/expr.cc: remove Wstrict-overflow from maybe_optimize_sub_cmp_0

Remove the single instance of Wstrict-overflow warning and the
associated testcase.

gcc/ChangeLog:

	* expr.cc (maybe_optimize_sub_cmp_0): Removed warning_at () call
	for Wstrict-overflow comparison.

gcc/testsuite/ChangeLog:

	* gcc.dg/Wstrict-overflow-25.c: Removed.