media: staging: rkisp1: remove unused field alloc_ctx from struct rkisp1_device

The field alloc_ctx in struct rkisp1_device is not used.
This patch removes it.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>