Intel: Remove partial Clang and DPC++ compiler support
Revert !5255 (merged) and !5300 (merged). The compilers have already been released, and are more usable with CMake by pretending to be upstream Clang than by identifying it as a compiler for which we have not implemented support.
This can be restored later in combination with more complete support for the compilers.
Fixes: #21551 (closed)
Issue: #21560, #21561 (closed)
Backport: release
Edited by Brad King