fix(mining-app): remove operator account note from expiration card

Remove the "运营账号贡献值永不失效" note from the contribution
expiration countdown card.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-app/lib/presentation/pages/contribution/contribution_page.dart

@@ -666,19 +666,6 @@ class ContributionPage extends ConsumerWidget {
                   '有效期 $validityDays 天',
                   style: const TextStyle(fontSize: 12, color: _orange, fontWeight: FontWeight.w500),
                 ),
-          const SizedBox(height: 8),
-          // 提示
-          Container(
-            padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 4),
-            decoration: BoxDecoration(
-              color: _bgGray,
-              borderRadius: BorderRadius.circular(4),
-            ),
-            child: const Text(
-              '* 运营账号贡献值永不失效',
-              style: TextStyle(fontSize: 10, color: _orange),
-            ),
-          ),
         ],
       ),
     );