Revert "Disable unused-extern-crates warnings for now"

This reverts commit 32462cd.
servo · bors-servo · Nov 8, 2018 · Nov 1, 2018 · Nov 5, 2018 · Nov 7, 2018
commit 50c840fe42f4bf91019e9e585ee9c1c85cde232e
diff --git a/python/servo/command_base.py b/python/servo/command_base.py
@@ -646,6 +646,8 @@ 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([