sysctl: clean up nr_pdflush_threads leftover

nr_pdflush_threads has been long deprecated and
removed, but a remnant of its glorious past is
still around in CTL_VM names enum. This patch
is a minor clean-up to that case.

Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>