The parameter innodb_prefix_index_cluster_optimization used to enable an
optimization that was added in cb37c557
and was disabled by default.

We will unconditionally enable the extension and mark the parameter
as deprecated.

Related to this, the counters
Innodb_secondary_index_triggered_cluster_reads and
Innodb_secondary_index_triggered_cluster_reads_avoided
allowed to determine the usefulness of this optimization.

Now that the configuration parameter is disabled, the counters
do not serve any useful purpose and can be removed.

row_search_with_covering_prefix(): Fix a bug that caused an
incorrect result to be returned.