Disable unused-extern-crates warnings for now

There are *many* new ones in rust 2018!
servo · bors-servo · Nov 6, 2018 · Nov 1, 2018 · Nov 1, 2018 · Nov 1, 2018
commit 99cd7ba3d460e6ae5a3f9614de4c817459bbf88a
diff --git a/python/servo/command_base.py b/python/servo/command_base.py
@@ -646,8 +646,6 @@ def package_dir(package):
             if target.startswith('arm') or target.startswith('aarch64'):
                 env['RUSTFLAGS'] = env.get('RUSTFLAGS', "") + " -C target-feature=+neon"
 
-        env['RUSTFLAGS'] = env.get('RUSTFLAGS', "") + " -W unused-extern-crates"
-
         git_info = []
         if os.path.isdir('.git') and is_build:
             git_sha = subprocess.check_output([