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

include/spl/sys/mutex.h

@@ -181,7 +181,4 @@ spl_mutex_lockdep_on_maybe(kmutex_t *mp)			\
 	/* NOTE: do not dereference mp after this point */	\
 }
 
-int spl_mutex_init(void);
-void spl_mutex_fini(void);
-
 #endif /* _SPL_MUTEX_H */

include/spl/sys/rwlock.h

@@ -196,10 +196,6 @@ RW_READ_HELD(krwlock_t *rwp)
 	downgrade_write(SEM(rwp));					\
 	spl_rw_lockdep_on_maybe(rwp);					\
 })
-
 /* END CSTYLED */
 
-int spl_rw_init(void);
-void spl_rw_fini(void);
-
 #endif /* _SPL_RWLOCK_H */