Remove TEST_IFUNC, tests-ifunc and *-ifunc.c tests.

Remove TEST_IFUNC, tests-ifunc and *-ifunc.c tests.

TEST_IFUNC is only tested in two headers, bench-string.h and
test-string.h, after it gets defined by those headers, and it never
gets undefined.

Thus no defines of TEST_IFUNC are needed, and the *-ifunc.c tests that
just define TEST_IFUNC and include other tests are also redundant, as
is the code to remove $(tests-ifunc) and $(xtests-ifunc) conditionally
from tests and xtests.  This patch removes the useless defines and
tests of TEST_IFUNC and the associated useless tests and makefile
code.  It thereby fixes a series of warnings
"../string/test-string.h:21:0: warning: "TEST_IFUNC" redefined" where
test-string.h defines TEST_IFUNC to empty, other files define it to 1
and this produces warnings.

Tested for x86_64.

	* debug/test-stpcpy_chk-ifunc.c: Remove file.
	* debug/test-strcpy_chk-ifunc.c: Likewise.
	* wcsmbs/test-wcschr-ifunc.c: Likewise.
	* wcsmbs/test-wcscmp-ifunc.c: Likewise.
	* wcsmbs/test-wcscpy-ifunc.c: Likewise.
	* wcsmbs/test-wcslen-ifunc.c: Likewise.
	* wcsmbs/test-wcsrchr-ifunc.c: Likewise.
	* wcsmbs/test-wmemcmp-ifunc.c: Likewise.
	* Rules [$(multi-arch) = no] (tests): Do not filter out
	$(tests-ifunc).
	[$(multi-arch) = no] (xtests): Do not filter out $(xtests-ifunc).
	* debug/Makefile (tests-ifunc): Remove variable.
	(tests): Do not add $(tests-ifunc).
	* wcsmbs/Makefile (tests-ifunc): Remove variable.
	(tests): Do not add $(tests-ifunc).
	* benchtests/bench-string.h (TEST_IFUNC): Remove macro.
	[TEST_IFUNC]: Remove conditionals.
	* string/test-string.h (TEST_IFUNC): Remove macro.
	[TEST_IFUNC]: Remove conditionals.