Align message content deletion experience with web app

We should make two changes to make the mobile app message deletion experience consistent with the web app and https://zulip.com/help/edit-or-delete-a-message#delete-a-message. Specifically, we should only use "Delete message" to refer to fully removing the message, rather than deleting message content.

We should:

• Drop the "Delete message" option from the menu for your own messages. (Once #4701 is resolved, this option will be re-added when the user has message deletion permissions.)
• Allow users to delete all the content of a message when editing messages. In that case, the message should be shown as "(deleted)". Currently, messaging editing changes cannot be saved when the resulting message content is empty.