msm: vidc: Remove implementation for V4L2_DEC_CMD_STOP

V4L2_DEC_CMD_STOP and V4L2_ENC_CMD_STOP are not needed
anymore. These commands were used to just send an event
back to userspace to unblock the polling thread. The
polling thread can be unblocked by using evenfds instead.

Change-Id: I3c3026a4d3dc46fe3eb97a79392401283a16b8fb
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>