core/blockchain: downgrade tx indexing and unindexing logs from info to debug

If a user has a finite tx lookup limit, they will see an "unindexing" info level log each time a block is imported. This information might help a user understand that they are removing the index each block and some txs may not be retrievable by hash, but overall it is generally more of a nuisance than a benefit. This change downgrades the log to a debug log.