Cleanup: Remove now-unused "tiled" compositor implementation

New ("fullframe") CPU compositor backend is being used now, and all the code
related to "tiled" CPU compositor is just never used anymore. The new backend
is faster, uses less memory, better matches GPU compositor, etc.

TL;DR: 20 thousand lines of code gone.

This commit:
- Removes various bits and pieces related to "tiled" compositor (execution
  groups, one-pixel-at-a-time node processing, read/write buffer operations
  related to node execution groups).
- "GPU" (OpenCL) execution device, that was only used by several nodes of
  the tiled compositor.
  - With that, remove CLEW external library too, since nothing within Blender
    uses OpenCL directly anymore.

Pull Request: https://projects.blender.org/blender/blender/pulls/118819

release/license/THIRD-PARTY-LICENSES.txt

@@ -32,7 +32,6 @@ https://github.com/AcademySoftwareFoundation/MaterialX
 ** meson; version 0.63 -- https://github.com/mesonbuild/meson
 ** oneAPI Threading Building Block; version 2020_U3 --
 https://software.intel.com/en-us/oneapi/onetbb
-** OpenCL Wrangler; version 27a6867 -- https://github.com/OpenCLWrangler/clew
 ** OpenImageDenoise; version 1.4.3 -- https://www.openimagedenoise.org/
 ** OpenImageIO; version 2.4.15.0 -- http://www.openimageio.org
 ** OpenSSL; version 3.1.2 -- https://www.openssl.org/