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>