Fix fallout from bits/string.h removal.

Remove one more string inline that was defined directly in string.h;
in the absence of the rest of the inlines, it broke the build.

Like other ifunc shims for these functions,
x86_64/multiarch/{mem,st}pcpy.c need to define __NO_STRING_INLINES and
NO_MEMPCPY_STPCPY_REDIRECT.

	* string/string.h (__mempcpy_inline): Delete.
	* sysdeps/x86_64/multiarch/mempcpy.c
	* sysdeps/x86_64/multiarch/stpcpy.c:
	Define NO_MEMPCPY_STPCPY_REDIRECT and __NO_STRING_INLINES
	before including string.h.