sql: Don't unconditionally clear all index zone configs when transitioning to REGIONAL BY ROW

[ajstorm]

Currently we drop all of the index zone configs when transitioning to REGIONAL BY ROW. The problem however, is if the user's created a zone configuration on one of the indexes to set a non-multi-region field, we'll blow away that change too. This issue should take a more subtle approach in dropZoneConfigsForMultiRegionIndexes and only clear out the zone configurations for indexes if they have multi-region fields set.

We'd want this test to work:

• create a RBR table with a secondary index
• set gc.ttlseconds and num_replicas on the index
• set override = true
• change locality to GLOBAL
• ensure the gc.ttlseconds is still on the index