media: amphion: Remove unused functions

The functions:
  vpu_color_get_default()
  vpu_color_check_full_range()
  vpu_color_check_primaries()
  vpu_color_check_transfers()
  vpu_color_check_matrix()

have been unused since 2022's
commit 1925665e ("media: amphion: remove redundant check of
colorspace in venc_s_fmt")

The (empty) function vpu_mbox_enable_rx() has been unused since it
was added in 2022 by the
commit 61cbf1c1 ("media: amphion: implement vpu core communication
based on mailbox")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>