c++: remove redundant warning [PR100879]

c++: remove redundant warning [PR100879]

Before my r277864, build_new_op promoted enums to int before passing them on
to cp_build_binary_op; after that commit, it doesn't, so
warn_for_sign_compare sees the enum operands and gives a redundant warning.
This warning dates back to 1995, and seems to have been dead code for a long
time--likely since build_new_op was added in 1997--so let's just remove it.

	PR c++/100879

gcc/c-family/ChangeLog:

	* c-warn.c (warn_for_sign_compare): Remove C++ enum mismatch
	warning.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/enum3.C: New test.