Deprecate and ignore the parameter innodb_prefix_index_cluster_optimization

The parameter innodb_prefix_index_cluster_optimization enables an optimization that was added in MDEV-6929. Should the optimization turn out to be incorrect and broken beyond repair in our stress testing, it will have to be reverted. If it is correct, there should be no point to disable it by default.

Related to this, the counters Innodb_secondary_index_triggered_cluster_reads and Innodb_secondary_index_triggered_cluster_reads_avoided are determining the usefulness of this optimization. If we hard-wire the optimization to enabled or disabled, these counters should not serve any useful purpose. As noted in MDEV-21212, frequently updating global counters is not for free.