hi846: remove digital gain v4l control

this makes sense because digital gain is now used for white balance
calibration. Overwriting it would undo that calibration.

there are others drivers too that expose analogue gain only, as does
the android driver that I loosely based this driver onto.

We never used digital gain in any application so far on the Librem 5
so this is no visible change.
2 jobs for hi846_white_balance in 64 minutes and 38 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #367534
librem5
build:aarch64

00:19:56

passed #367535
librem5:arm64
build:deb

01:04:38