drivers/dax: 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: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: <nvdimm@lists.linux.dev>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>

drivers/dax/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 menuconfig DAX
 	tristate "DAX: direct access to differentiated memory"
-	select SRCU
 	default m if NVDIMM_DAX
 
 if DAX