test: add remove() no-labels guard test per codex review

Codex flagged that parallel tests for remove() on label-less metrics
strengthen the fix. Also removed unnecessary test docstring.

Author: kimjune01

tests/test_core.py

@@ -621,11 +621,15 @@ def test_clear(self):
         self.assertEqual(None, self.registry.get_sample_value('c_total', {'l': 'y'}))
 
     def test_clear_no_labels_raises(self):
-        """clear() on a metric without labels should raise ValueError, not AttributeError."""
         no_labels = Counter('c_no_labels', 'help', registry=self.registry)
         no_labels.inc()
         self.assertRaises(ValueError, no_labels.clear)
 
+    def test_remove_no_labels_raises(self):
+        no_labels = Counter('c_no_labels2', 'help', registry=self.registry)
+        no_labels.inc()
+        self.assertRaises(ValueError, no_labels.remove, 'x')
+
     def test_incorrect_label_count_raises(self):
         self.assertRaises(ValueError, self.counter.labels)
         self.assertRaises(ValueError, self.counter.labels, 'a', 'b')