This was added in early 2013 and abandoned several months later; as far as
I can tell, there are no external users.  Future OpenCL use will be via
hwcontext, which requires neither special OpenCL-only API nor global state
in libavutil.

All internal users are also deleted - this is just the unsharp filter
(replaced by unsharp_opencl, which is more flexible) and the deshake filter
(no replacement).