V4L/DVB (5968): videodev2.h: remove superfluous FBUF GLOBAL_INV_ALPHA support

There is no need for a global inverted alpha capability since all the
application has to do is to pass '255-alpha' as the global alpha value.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>