media: staging/intel-ipu3: remove the unnecessary compiler flags

Currently, we can build ipu3 driver code without any
warnings with W=1, so the extra compiler flags in Makefile
and the item in TODO file can be removed.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>