Remove dead files.

memcpy.a15.S/strcmp.a15.S files were submitted by ARM for use as the basis
for the memcpy/strcmp implementations in cortex-a15.

memset.S was moved in to the generic directory.

NOTE: memcpy.a9.S was submitted by Linaro to be the basis for the memcpy
for cortex-a9/cortex-a15 but has not been incorporated yet.

Bug: 10971279

Change-Id: Id8d97c888b4b9951cf49e1d94902501960720d56