media: platform: exynos4-is: Remove unused __is_get_frame_size

The last use of __is_get_frame_size() was removed in 2013 by
commit 5cfaad64 ("[media] exynos4-is: Fix format propagation on
FIMC-IS-ISP subdev")

Remove it.

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