media: ext-ctrls-codec.rst: move FWHT docs to ext-ctrls-codec-stateless.rst

Move the documentation for the stateless FWHT codec to
ext-ctrls-codec-stateless.rst since that is the proper place
for the stable stateless API documentation.

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