Clean up predictive_back_priority_system_navigation_observer flag

This CL removes all references to the flag. The aconfig flag definition
itself is not removed due to isExported=true.

Bug: 436710530
Bug: 362938401
Test: presubmit
Flag: EXEMPT flag cleanup
Change-Id: I3a06a5ab925a688ca812a8014715db8d760a6d9b