llvm: remove versioning

It was decided[1] to keep only single llvm version in Sdk. We may introduce second llvm for rust/mesa but it will be put in extensions subdirectory and not overlap with one from Sdk. That means we don't need versioning llvm anymore.

Removing versioning will also make future updates of llvm easier without toching all alements that depend on it. It will also make our filename API more stable across releases[2].

[1] #488 (closed) [2] #432 (closed)