redundant index rewards_by_coinbase on table rewards

[neurosis69]

The table rewards is created as WITHOUT ROWID which is creating clustered index on the primary key implicitely.
To save storage and computing overhead it would make sense to remove the index rewards_by_coinbase.

go-spacemesh/sql/migrations/0001_initial.sql

Line 60 in 9c5e88b

CREATE INDEX rewards_by_coinbase ON rewards (coinbase, layer);