clang: fix reproducibility issue in LLVMConfig.cmake

clang: fix reproducibility issue in LLVMConfig.cmake

The LLVMConfig.cmake file is mostly LLVM configuration, but it also
specifies if some Python modules (needed by the opt-viewer tool) were
present on the build host.

This is host contamination and a source of non-determinism, so remove it
from the installed file.

A ticket has been filed upstream to resolve this:

  https://github.com/llvm/llvm-project/issues/161199

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>