powerpc: Remove power7 strstr optimization

The optimization is not faster than the generic algorithm,
using the bench-strstr the geometric mean running on a POWER10 machine
using gcc 13.1.1 is 482.47 while the default __strstr_ppc is 340.97
(which uses the generic implementation).

Also, there is no need to redirect the internal str*/mem* call
to optimized version, internal ifunc is supported and enabled
for internal calls (meaning that the generic implementation
will use any asm optimization if available).

Checked on powerpc64le-linux-gnu.
Reviewed-by: Peter Bergner <bergner@linux.ibm.com>