[FIX] account_financial_report: Avoid slow vacuum due to ondelete=cascade

`report_journal_ledger` is auto-vacuumed as any transient model, but has some
ondelete="cascade" constraints that auto-remove subtables when a record is removed,
doing this operation very slow when selecting these sub-records.

Letting default ondelete="set null" would result in same performance bottleneck,
as the select on sub-table is performed the same for setting "null" value on them.

As a solution, and for avoiding a costly index operation, we delete by SQL sub-tables
rows in advance.

A bit of extra logic has been added for avoiding to remove that records if it's not
the turn of vacuum the parent table.

account_financial_report/__manifest__.py

@@ -4,7 +4,7 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 {
     'name': 'Account Financial Reports',
-    'version': '12.0.1.2.1',
+    'version': '12.0.1.2.2',
     'category': 'Reporting',
     'summary': 'OCA Financial Reports',
     'author': 'Camptocamp SA,'