Rollup merge of #146466 - klensy:llvm-wrapper-c, r=cuviper

llvm-wrapper: other cleanup

Cleanup few things around llvm-wrapper:

First commit removes unused macro, added back in rust-lang/rust#31709 it was used in `LLVMRustHasFeature`; second removes unused function, third one removes `jsbackend` llvm component - this is looks like remnants of some removed backend?

Author: Zalathar