Schema: Drop unused and useless indexes of sites table

This table has eight indexes plus PK. It has around 1000 rows only. Even
if it needs these indexes (which it doesn't), they are still useless.

Looking at the code, the only potential useful index is the one on
site_global_key, they are showing up in the report of unused indexes in
the db and I checked with Fandom (which might benefit from an index on
this table) and they said they don't use sites table.

Bug: T342856
Change-Id: I06b3db0f33bd35bfa68f4b418d8c2f4b9b988409