Remove the option to disable llvm-version-check

We don't support old versions of LLVM; there's no reason to have an easy
way to force bootstrap to use them anyway. If someone really needs to
use an unsupported version, they can modify bootstrap to change the
version range.

src/bootstrap/native.rs

@@ -549,10 +549,6 @@ fn get_built_llvm_lib_path(llvm_config_path: &Path) -> PathBuf {
 }
 
 fn check_llvm_version(builder: &Builder<'_>, llvm_config: &Path) {
-    if !builder.config.llvm_version_check {
-        return;
-    }
-
     if builder.config.dry_run() {
         return;
     }