postgres: Make the upgrade ignore dropping indexes that might not exist
Due to drifts, 1.35 didn't have tl_namespace nor tl_backlinks_namespace
but that was fixed in 1.36 which the drop of tl_title tries to drop
leading to errors.
Bug: T330382
Change-Id: I555e855b3c0e97e5df1f0fab1de104301d40eae4