[clang] [llvm] [cmake] Remove custom linker flag check function (PR #86602)

[Tom Stellard via cfe-commits]

tstellar wrote:

@petrhosek Would you be able to add a comment to the llvm_check_linker_flag function explaining this so someone else doesn't try to do the same thing.

https://github.com/llvm/llvm-project/pull/86602