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