Revert migration adding annotation.user_id

Investigating performance problems in H's DB. Trying to rule this table
changes as the direct cause.

h/migrations/versions/6df1c8c3e423_revert_annotation_user_id.py

@@ -0,0 +1,18 @@
+"""Revert annotation.user_id."""
+from alembic import op
+
+revision = "6df1c8c3e423"
+down_revision = "076ac6c7a8e0"
+
+
+def upgrade():
+    op.drop_index("ix__annotation_user_id", table_name="annotation")
+    op.drop_constraint(
+        "fk__annotation__user_id__user", "annotation", type_="foreignkey"
+    )
+    op.drop_column("annotation", "user_id")
+
+
+def downgrade():
+    """No downgrade, see the upgrade for migration 8250dce465f2."""
+    pass