Remove dead code from ZenModeController

While we should move away from ZenModeController altogether, some parts of it are already unused (and if we get closer to deleting getConfig and its callback, we can save a lot of Binder traffic).

More deletions will be possible when MODES_UI is inlined.

Bug: 308591859
Test: build
Flag: EXEMPT Removing dead code
Change-Id: I97b59392a8c50b0a25ff07890b068e4e03140887