powerpc: Remove powerpc64 strncmp variants

The default, and power7 implementation just adds word aligned
access when inputs have the same aligment.  The unaligned case
is still done by byte operations.

This is already covered by the generic implementation, which also add
the unaligned input optimization.

Checked on powerpc64-linux-gnu built without multi-arch for powerpc64,
power7, power8, and power9 (build for le).
Reviewed-by: Rajalakshmi Srinivasaraghavan <rajis@linux.ibm.com>