mysys: remove optimized memcpy from 18 years ago

While this code has remained dormant for 18 years, libc implementers have used assembly features to gain improvements using architecture features optimized and by the buffer length like:

https://svnweb.freebsd.org/base/head/lib/libc/amd64/string/memcmp.S
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/aarch64/memcmp.S
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/powerpc/powerpc64/memcpy.S
While the perf results in commit message don't show a great improvement the varying length and codebase shows optimizations based on length.

Pull request: https://github.com/MariaDB/server/pull/698