Remove llvm.skip-rebuild option

This was added to in 2019 to speed up rebuild times when LLVM was
modified. Now that download-ci-llvm exists, I don't think it makes sense
to support an unsound option like this that can lead to miscompiles; and
the code cleanup is nice too.

src/bootstrap/native.rs

@@ -109,15 +109,6 @@ pub fn prebuilt_llvm_config(
     let stamp = out_dir.join("llvm-finished-building");
     let stamp = HashStamp::new(stamp, builder.in_tree_llvm_info.sha());
 
-    if builder.config.llvm_skip_rebuild && stamp.path.exists() {
-        builder.info(
-            "Warning: \
-                Using a potentially stale build of LLVM; \
-                This may not behave well.",
-        );
-        return Ok(res);
-    }
-
     if stamp.is_done() {
         if stamp.hash.is_none() {
             builder.info(