IntelLLVM: Remove unsupported -imsvc system include flag

`clang-cl` supports the `-imsvc` flag to tell the compiler an include
directory is intended for system paths.  `icx` does not accept this
flag, even on MSVC platforms, so do not tell CMake that it exists.

Fixes: #21801
Signed-off-by: william.r.dieter <[email protected]>

Modules/Compiler/IntelLLVM.cmake

@@ -18,7 +18,6 @@ set(__pch_header_OBJCXX "objective-c++-header")
 if(CMAKE_HOST_WIN32)
   # MSVC-like
   macro(__compiler_intel_llvm lang)
-    set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-imsvc ")
     if(NOT "x${lang}" STREQUAL "xFortran")
       set(CMAKE_${lang}_COMPILE_OPTIONS_INVALID_PCH -Winvalid-pch)
     endif()