Remove empty imacfb.c file

Commit 7c08c9ae ("efifb/imacfb
consolidation + hardware support") claimed to remove imacfb entirely and
merge its DMI table into the efifb driver.  So far so good, but the diff
actually ended up just generating an empty file instead of removing it.

[ Technical reason: the patch header looked like

  diff -puN drivers/video/imacfb.c~efifb-imacfb-consolidation-hardware-support drivers/video/imacfb.c
  --- a/drivers/video/imacfb.c~efifb-imacfb-consolidation-hardware-support
  +++ a/drivers/video/imacfb.c
  @@ -1,376 +0,0 @@

  which git will think is a truncation, not a delete.  Git wants to see a
  target of /dev/null to consider it a delete. ]

So remove it properly.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>