Remove index named after alias.

If you pass --rebuild to updateSearchIndexConfig.php it'll remove any
index that exists with a name equal to the alias.

We could do this all the time but deleting things is kind of destructive
so I paired it with --rebuild which is very destructive.

Bug: 54481

Change-Id: Ie1bead3286d092b8548ac22c6f2afbbba13ca073
Merge "Remove index named after alias."