Remove the ZenModeSliceBuilder

We don't want the old Do Not Disturb page to show up in the search index
results, so we can remove the custom Slice Builder.

Note that this is part of the general Do Not Disturb/Modes Settings Changes: see b/353700470

Bug: 341726633
Test: atest SettingsSliceProviderTest, flash+test
Flag: android.app.modes_ui
Change-Id: Ibba372d5fa6caf3ee28d9fd2a900047060b212da