[lib] Rip useShouldShowDisconnectedBar and useDisconnectedBar out of hooks/disconnected-bar

Summary:
These hooks are no longer relevant since DisconnectedBar is now about displaying information about network connectivity, not keyserver connectivity.

Depends on D10475

Test Plan: These functions are no longer used.

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Subscribers: inka

Differential Revision: https://phab.comm.dev/D10476