[Project cleaning] Remove unused Tchap source files

[giomfo]

We moved back several modules on the Element source code during the previous months
I request here to remove definitely unused Tchap source code (from Tchap sub folder)
Please create an issue for each module (feature) which was not moved back to Element yet in order to take a decision on them

[phloux]

After these changes, there is still Tchap specific code for those parts :

• Authentication / Account creation
• Change Password / Forgot Password
• Account Validity
• Password Policy
• App version checker
• Favorites Messages -> to be removed by a specific PR (Tchap will wait for Element implementation on favorite messages if any)
• Global Search -> should be removed with Home manager? -> Removed
• Avatar -> should be removed by adjusting some codes (like room access by link view model) -> Removed
• Home -> should be removed now? -> Removed
• Forward -> should be removed -> Removed
• Public Rooms
• Room Creation
• Room Preview -> @phlniji can you please check whether we can remove it? -> Not removable: Used by Public Rooms
• 3PID -> @phlniji can you please check whether we can remove it? -> Not removable: Used by UserService
• Media Service -> @phlniji can you please check whether we can remove it? -> Not removable: Used by Room Creation
• Invite by email
• Identity Service
• Home Server
• Display Name

[giomfo]

We will track the pending list with #663