media: dev-effect.rst: remove unused Effect Interface chapter

We never had an effect interface, and if you want to do such
things you use the mem2mem interface instead.

Just drop this from the spec.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>