cvsobel han't been working for a while due to a silent change in OpenCV
API. It would return a plain black image. This commit updates the usage
of cvSobel by using cvCvtColor to create the grayscale image to process.
This also means there is no need to use GstBaseTransform's transform_caps
anymore, since the pads can be RGB.
The opencv element includes were full of duplicates and uneeded headers.
For example a few elements that stopped using gstcvopencvutils still
included that header file.
