Bluetooth: AICS: Remove bad check in handle_set_gain_op

AICS gain mode does not prevent local nor remote operations from changing
the gain setting value. The gain setting value shall simply be ignored if
in the BT_AICS_MODE_AUTO_ONLY or BT_AICS_MODE_AUTO modes, but the local
application and remote devices can still modify the gain setting value.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>