powerpc: Remove duplicate strchrnul and strncasecmp_l libc.a (BZ 31786)

For powerpc64 the generic version provides a weak definition of
strchrnul, which are already provided by the ifunc resolver.  The
powerpc32 version is slight different, where for static case there
is no iFUNC support.

The strncasecmp_l is provided ifunc resolver.

Checked on powerpc-linux-gnu-power4 and powerpc64-linux-gnu.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>