[spanner] Reevaluate the BsoModified index

[pjenvey]

The secondary index on the modified column doesn't get nearly as much use as we intended. Let's reevaluate its usefulness, because it may not be worth keeping.

Removing it can saves a small number of mutations and storage space, and though likely a long shot, might provide a small write speed improvement to the migration.

[pjenvey]

Apparently BsoModified is a lot larger than we thought, so removing it could save us a significant amount of storage space/cost.

[pjenvey]

As does BsoExpired, which should also be reevaluated, particularly on how necessary it would be if we were to take advantage of Spanner's native TTL support.