[MERGE][REF] mail, various: remove dead code and perform some small code renaming and optimization

Purpose of this merge is to clean a bit various methods and remove some
dead code. All those commits come from other work-in-progress branches
in mail aiming at improving performances and reducing code size. We can
already merge some light cleaning in order to lessen diff and ease
understanding of real feature-based branches.

It contains

 * some renaming in notification methods to match the _notify prefix used
   in most of them;
 * removal of  some dead code coming from old implementations of chatter;
 * removal of unnecessary context switch in email sending process;
 * small cleaning of notification group methods in various addons;

It allows to gain some queries in various test_mail performance test as
stated in the test_performance file update. When notification emails are
sent 4 queries are gained with this merge. On the whole community runbot
the impact is small (about 1K queries) probably because most of email
sending part is skipped on runbot.

This merge is related to the task Id 59039. #Closes #23089.