media: remove unused variable that causes a warning

My 'allmodconfig' build is _almost_ free of warnings, and most of the
remaining ones are for legacy drivers that just do bad things that I
can't find it in my black heart to care too much about.  But this one
was just annoying me:

   drivers/media/v4l2-core/videobuf2-core.c:3256:26: warning: unused variable ‘fileio’ [-Wunused-variable]

because commit 0e661006 ("[media] vb2: fix 'UNBALANCED' warnings
when calling vb2_thread_stop()") removed all users of 'fileio' and
instead calls "__vb2_cleanup_fileio(q)" to clean up q->fileio.  But the
now unused 'fileio' variable was left around.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>