[native] Unify alert behavior when changing selected edited message

Summary:

• removed confirmation alert, when changing edited message and didn't make changes to the previous message,
• removed the Edit button for the currently edited message.

Test Plan:
Tested on iOS:

• entered edit mode,
• tried to edit the same message, checked if the edit button doesn't appear,
• made some changes in the edited message,
• tried to edit another message, checked if the confirmation alert is displayed,
• entered edit mode, didn't make changes, checked if can select another message to edit without confirmation alert,

Reviewers: michal, inka

Reviewed By: inka

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D8036