Revert "Intel: Add Intel DPC++ compiler identification"

Revert commit 887f3a88a6 (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

Help/variable/CMAKE_LANG_COMPILER_ID.rst

@@ -25,7 +25,6 @@ include:
   HP = Hewlett-Packard Compiler (hp.com)
   IAR = IAR Systems (iar.com)
   Intel = Intel Compiler (intel.com)
-  IntelDPCPP = Intel DPCPP Compiler (intel.com)
   MSVC = Microsoft Visual Studio (microsoft.com)
   NVIDIA = NVIDIA CUDA Compiler (nvidia.com)
   OpenWatcom = Open Watcom (openwatcom.org)

Modules/CMakeCompilerIdDetection.cmake

@@ -78,7 +78,6 @@ function(compiler_id_detection outvar lang)
       ARMCC
       AppleClang
       ARMClang
-      IntelDPCPP
       Clang
       GNU
       MSVC

Modules/Compiler/IntelDPCPP-DetermineCompiler.cmake

@@ -1,7 +0,0 @@
-
-set(_compiler_id_pp_test "defined(__clang__) && defined(__INTEL_DPCPP_COMPILER__)")
-
-include("${CMAKE_CURRENT_LIST_DIR}/Clang-DetermineCompilerInternal.cmake")
-
-string(APPEND _compiler_id_version_compute "
-# define @PREFIX@COMPILER_VERSION_TWEAK @MACRO_DEC@(__INTEL_DPCPP_COMPILER__)")