[3.1.x] Fixed #31965 -- Adjusted multi-table fast-deletion on MySQL/MariaDB.

The optimization introduced in 7acef095d7 did not properly handle
deletion involving filters against aggregate annotations.

It initially was surfaced by a MariaDB test failure but misattributed
to an undocumented change in behavior that resulted in the systemic
generation of poorly performing database queries in 5b83bae031.

Thanks Anton Plotkin for the report.

Refs #23576.

Backport of f6405c0b8e from master

tests/delete/tests.py

@@ -709,3 +709,16 @@ class FastDeleteTests(TestCase):
         referer = Referrer.objects.create(origin=origin, unique_field=42)
         with self.assertNumQueries(2):
             referer.delete()
+
+    def test_fast_delete_aggregation(self):
+        # Fast-deleting when filtering against an aggregation result in
+        # a single query containing a subquery.
+        Base.objects.create()
+        with self.assertNumQueries(1):
+            self.assertEqual(
+                Base.objects.annotate(
+                    rels_count=models.Count('rels'),
+                ).filter(rels_count=0).delete(),
+                (1, {'delete.Base': 1}),
+            )
+        self.assertIs(Base.objects.exists(), False)