cvlaplace was also affected by the silent change in OpenCV API, same as
cvsobel. It hasn't been working for a while. It would return a plain black
image. This commit updates the usage of cvLaplace by using cvCvtColor to
create the grayscale intermediate image to process. This also means there
is no need anymore to use GstBaseTransform's transform_caps, since the pads
are 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.
