media: platform: Delete redundant code and add annotation for an enum

Delete unused member variables annotation.
Delete unused variable definition.
Delete redundant log print, because V4L2 debug logs already print it.
Add annotation for enum mtk_jpeg_ctx_state.

Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Xia Jiang <xia.jiang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>