drivers/md: Remove "select SRCU"

Now that the SRCU Kconfig option is unconditionally selected, there is
no longer any point in selecting it.  Therefore, remove the "select SRCU"
Kconfig statements.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Cc: <dm-devel@redhat.com>
Cc: <linux-raid@vger.kernel.org>
Acked-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: John Ogness <john.ogness@linutronix.de>