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 <william.r.dieter@intel.com>
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