[MERGE][REM] mail, various: remove mail-related dead or unnecessary code

Purpose of this merge is to remove
 * dead code related to mail features, like unnecessary overrides;
 * unnecessary subscription or message post that adds no value to the app
   while creating noise;
 * some deprecated code bits in mail;

Please see subcommits for more details. This merge is related to task
ID 61132. Closes #23624 .