Retire unused spl_{mutex,rwlock}_{init_fini}

Retire unused spl_{mutex,rwlock}_{init_fini}

These functions are unused and can be removed along
with the spl-mutex.c and spl-rwlock.c source files.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9029