[llvm] docs: remove STRIP_FROM_INC_PATH in Doxygen to avoid incorrect source file linking (PR #140114)

[via llvm-commits]

cor3ntin wrote:

Thanks for working on that.

We have several `doxygen.cfg.in` files in various places in the repo for different subprojects. They should probably be consistent


https://github.com/llvm/llvm-project/pull/140114