Revert "media: subdev: Add v4l2_subdev_is_streaming()"

This reverts commit a64a102e which is
commit 5f3ce14f upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I1a6143299ed8fea16154f6ca0c7e223b96846e2e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>