s390/kasan: remove override of mem*() functions

The kasan mem*() functions are not used anymore since s390 has switched
to GENERIC_ENTRY and commit 69d4c0d3 ("entry, kasan, x86: Disallow
overriding mem*() functions").

Therefore remove the now dead code, similar to x86.
While at it also use the SYM* macros in mem.S.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>