Cleanup flag remember_view_model_off_main_thread

This flag is no longer needed.

The code was cleaned in https://googleplex-android-review.git.corp.google.com/q/topic:%22rememberViewModel-bgThread-revert%22

Fix: 421000550
Test: N/A flag removal
Flag: EXEMPT flag removal
Change-Id: I85505f31fd5bceeeb9ed2279b6db11df9aa38792