Remove pool io kstats

This mostly reverts "3537 want pool io kstats" commit of 8 years ago.

From one side this code using pool-wide locks became pretty bad for
performance, creating significant lock contention in I/O pipeline.
From another, there are more efficient ways now to obtain detailed
statistics, while this statistics is illumos-specific and much less
usable on Linux and FreeBSD, reported only via procfs/sysctls.

This commit does not remove KSTAT_TYPE_IO implementation, that may
be removed later together with already unused KSTAT_TYPE_INTR and
KSTAT_TYPE_TIMER.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12212

lib/libzpool/kernel.c

@@ -146,36 +146,6 @@ void
 kstat_delete(kstat_t *ksp)
 {}
 
-/*ARGSUSED*/
-void
-kstat_waitq_enter(kstat_io_t *kiop)
-{}
-
-/*ARGSUSED*/
-void
-kstat_waitq_exit(kstat_io_t *kiop)
-{}
-
-/*ARGSUSED*/
-void
-kstat_runq_enter(kstat_io_t *kiop)
-{}
-
-/*ARGSUSED*/
-void
-kstat_runq_exit(kstat_io_t *kiop)
-{}
-
-/*ARGSUSED*/
-void
-kstat_waitq_to_runq(kstat_io_t *kiop)
-{}
-
-/*ARGSUSED*/
-void
-kstat_runq_back_to_waitq(kstat_io_t *kiop)
-{}
-
 void
 kstat_set_raw_ops(kstat_t *ksp,
     int (*headers)(char *buf, size_t size),