s390/string: Remove strcpy() implementation

Remove the optimized strcpy() library implementation. This doesn't make any
difference since gcc recognizes all strcpy() usages anyway and uses the
builtin variant. There is not a single branch to strcpy() within the
generated kernel image, which also seems to be the reason why most other
architectures don't have a strcpy() implementation anymore.

Reviewed-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>