rustbuild: Remove --enable-llvm-clean-rebuild

This was intended for bots back in the day where we'd persist caches of LLVM
builds across runs, but nowadays we don't do that on any of the bots so this
option is no longer necessary

src/bootstrap/native.rs

@@ -80,9 +80,6 @@ impl Step for Llvm {
                 return
             }
         }
-        if build.config.llvm_clean_rebuild {
-            drop(fs::remove_dir_all(&out_dir));
-        }
 
         let _folder = build.fold_output(|| "llvm");
         println!("Building LLVM for {}", target);