sysctl: remove /proc/sys/vm/nr_pdflush_threads

This tunable has been obsolete since 2.6.32, and writes to the
file have been failing and complaining in dmesg since then:

nr_pdflush_threads exported in /proc is scheduled for removal

That was 8 years ago. Remove the file ABI obsolete notice, and
the sysfs file.
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/ABI/obsolete/proc-sys-vm-nr_pdflush_threads

-What:		/proc/sys/vm/nr_pdflush_threads
-Date:		June 2012
-Contact:	Wanpeng Li <liwp@linux.vnet.ibm.com>
-Description: Since pdflush is replaced by per-BDI flusher, the interface of old pdflush
-             exported in /proc/sys/vm/ should be removed.

kernel/sysctl.c

@@ -1344,11 +1344,6 @@ static struct ctl_table vm_table[] = {
 		.proc_handler	= dirtytime_interval_handler,
 		.extra1		= &zero,
 	},
-	{
-		.procname       = "nr_pdflush_threads",
-		.mode           = 0444 /* read-only */,
-		.proc_handler   = pdflush_proc_obsolete,
-	},
 	{
 		.procname	= "swappiness",
 		.data		= &vm_swappiness,