media: atomisp: gc0310: Remove read-only exposure control

The sensor does not have automatic exposure so the exposure cannot
change underneath us.

And the control is not marked volatile in it flags,
so the .g_volatile_ctrl callback will never get called.

Remove the current broken read-only (no s_ctrl implementation)
exposure ctrl, this will be replaced with a proper r/w exposure
control relying on the kernel caching the last set value
(so non volatile).

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>