media: v4l2-subdev.rst: get rid of a duplicatd kernel-doc markup

There are two kernel-doc markups for include/media/v4l2-async.h,
one at v4l2-async.rst and another one at v4l2-subdev.rst.

Sphinx 3.x checks it and complains for duplicated symbols.

So, get rid of one of them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>