Clean up confusing OpenCL code a bit

Get rid of the silly OpenCLDevice class that had only static members. We can
as well just use namespacing. Remove functions only used internally in
openclwrapper.cxx from the now public openclwrapper.hxx header.

Change-Id: If7336edd262c772564dc13e64113d72d0b52428c
The OpenCLEnv struct is used only inside openclwrapper.cxx

Change-Id: I95fbee302213e6ced469dd7a2a1c254178159357
Put the KernelEnv struct inside the opencl namespace, too

Change-Id: I33d51b0e30445c34c792210e7c656c1d48159019