[4.2.x] Fixed #34304 -- Made MySQL's SchemaEditor.remove_constraint() don't create foreign key index when unique constraint is ignored.

Regression in b731e88415.
Backport of 110b3b8356 from main

django/db/backends/mysql/schema.py

@@ -121,7 +121,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             )
 
     def remove_constraint(self, model, constraint):
-        if isinstance(constraint, UniqueConstraint):
+        if (
+            isinstance(constraint, UniqueConstraint)
+            and constraint.create_sql(model, self) is not None
+        ):
             self._create_missing_fk_index(
                 model,
                 fields=constraint.fields,

tests/schema/tests.py

@@ -2676,6 +2676,37 @@ class SchemaTests(TransactionTestCase):
         with self.assertRaises(IntegrityError):
             Tag.objects.create(title="bar", slug="foo")
 
+    def test_remove_ignored_unique_constraint_not_create_fk_index(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(Book)
+        constraint = UniqueConstraint(
+            "author",
+            condition=Q(title__in=["tHGttG", "tRatEotU"]),
+            name="book_author_condition_uniq",
+        )
+        # Add unique constraint.
+        with connection.schema_editor() as editor:
+            editor.add_constraint(Book, constraint)
+        old_constraints = self.get_constraints_for_column(
+            Book,
+            Book._meta.get_field("author").column,
+        )
+        # Remove unique constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(Book, constraint)
+        new_constraints = self.get_constraints_for_column(
+            Book,
+            Book._meta.get_field("author").column,
+        )
+        # Redundant foreign key index is not added.
+        self.assertEqual(
+            len(old_constraints) - 1
+            if connection.features.supports_partial_indexes
+            else len(old_constraints),
+            len(new_constraints),
+        )
+
     @skipUnlessDBFeature("allows_multiple_constraints_on_same_fields")
     def test_remove_field_unique_does_not_remove_meta_constraints(self):
         with connection.schema_editor() as editor: