Delete Settings code related to super-obsolete zen onboarding

A notification was shown when upgrading from pre-P to P because default DND settings had changed. The default was later changed back so it was already obsolete at that point. Moreover, it should be impossible to upgrade from pre-P to V in one step.

Bug: 346789065
Test: N/A
Flag: EXEMPT Unflaggable -- removing resources, manifest entries, etc.
Change-Id: I2862211dc9709b741e60beca5fc029cc054745a3