mtd: ubi: Remove unused ubi_flush

ubi_flush() was added in 2012 as part of
commit 62f38455 ("UBI: modify ubi_wl_flush function to clear work queue
for a lnum")
but has remained unused.

(It's friend ubi_wl_flush() is still used)

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Richard Weinberger <richard@nod.at>