Remove powerpc64 strspn, strcspn, and strpbrk implementation

This patch removes the powerpc64 optimized strspn, strcspn, and
strpbrk assembly implementation now that the default C one
implements the same strategy.  On internal glibc benchtests
current implementations shows similar performance with -O2.

Tested on powerpc64le (POWER8).

	* sysdeps/powerpc/powerpc64/strcspn.S: Remove file.
	* sysdeps/powerpc/powerpc64/strpbrk.S: Remove file.
	* sysdeps/powerpc/powerpc64/strspn.S: Remove file.