Remove unnecessary index lookups for single partition reads

For single partition reads, we currently perform an unnecessary call to SecondaryIndexManager to lookup the most selective index based on the command's RowFilter. This is redundant as if any applicable indexes are present, the query would be performed using a range command (there was some code introduced into CassandraIndexSearcher by CASSANDRA-8099 in preparation for supporting indexes on single partition reads, but it is as yet unused). Given that, we should make sure we don't perform that lookup for now.