Do not commit readonly transaction even with channel

Even if the backend allows NOTIFY query on readonly transaction, it is strange
to commit readonly transaction because the backend may not enforce the
readonly.
Also commit performs other actions that may not be wanted like commit
datamanager or clear cache.
A procedure that requires to perform a NOTIFY should have a non-readonly
transaction.

Closes #12542

trytond/trytond/transaction.py

@@ -231,9 +231,7 @@ class Transaction(object):
             if transactions.count(self) == 1:
                 try:
                     try:
-                        # Transaction must be commited to send notifications
-                        if commit and (not self.readonly
-                                or self.database.has_channel()):
+                        if commit and not self.readonly:
                             self.commit()
                         else:
                             self.rollback()