Revert "Intel: Add Intel DPC++ compiler identification"
Revert commit 887f3a88 (Intel: Add Intel DPC++ compiler identification, 2020-09-21, v3.19.0-rc1~124^2). The compiler has already been released, and is more usable with CMake by pretending to be upstream Clang than by identifying it as a compiler for which we have not implemented support. Fixes: #21551