Fix error during direct DB upgrade from v2 to v3.3

We used the same `merge_map` temp table in two revisions, but since
there is no commit in between, it's still around when the second
revision runs. We now `DROP` it explicitly right after using it to
avoid this. Creating it `IF NOT EXISTS` would have been an alternative
options, but the benefit is minimal and alembic revisions should be
self-contained.

indico/migrations/versions/20210607_1548_735dc4e8d2f3_update_merged_users_in_registrations.py

@@ -41,6 +41,8 @@ def upgrade():
             JOIN merge_map mm2 ON (mm2.user_id = r3.user_id AND mm2.current_user_id = mm.current_user_id)
             WHERE r3.registration_form_id = r.registration_form_id AND mm2.user_id != r.user_id
         );
+
+        DROP TABLE merge_map;
     ''')
 
 

indico/migrations/versions/20240206_1223_5bb555dd91eb_update_merged_users_in_abstract_email_logs.py

@@ -30,6 +30,8 @@ def upgrade():
         SET user_id = mm.current_user_id
         FROM merge_map mm
         WHERE mm.user_id = eml.user_id AND mm.current_user_id != eml.user_id;
+
+        DROP TABLE merge_map;
     ''')