[native] Remove background option from sidebar settings
Summary:
Part of ENG-1729
Sidebars are always displayed under their parent chat, so the option of moving them between Focused and Background does nothing and should be removed.
Test Plan:
- Check if the option is no longer visible in sidebar settings
- Check if it's still visible and works correctly in other chats
Reviewers: tomek, atul, abosh, ashoat
Reviewed By: ashoat
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D5254