Don't allow using unstable features in bootstrap itself

This shouldn't have any change in practice, but it seems good to enforce.

Author: jyn514

src/bootstrap/bootstrap.py

@@ -1246,7 +1246,6 @@ def bootstrap(help_triggered):
     env["BOOTSTRAP_PARENT_ID"] = str(os.getpid())
     env["BOOTSTRAP_PYTHON"] = sys.executable
     env["BUILD_DIR"] = build.build_dir
-    env["RUSTC_BOOTSTRAP"] = '1'
     if toml_path:
         env["BOOTSTRAP_CONFIG"] = toml_path
     if build.rustc_commit is not None: