Remove setting 'when I delete a message'

Remove the setting 'When I delete a message' and always delete a message from the server when it is deleted locally.

Since this is a behavior change for people who don't use the default value of "Delete from server" we need to inform those users about the change. I propose to show a dialog when users try to delete a message letting them know that the feature was removed and that the message will be deleted from the server if they choose to continue. The dialog should contain a checkbox the user can check to no longer receive this warning in the future.
It's probably a good idea to skip the delete confirmation dialog if this warning dialog is enabled. Otherwise users will have to confirm the deletion of a message twice.

This is part of the effort to remove support for local messages.

No files, tests, or entry points are named in the issue. Trace the “When I delete a message” setting and the local message deletion flow, then verify that deletion always reaches the server, affected users see a one-time warning with a persistence checkbox, and redundant confirmation is avoided.