stop checking if the generated plan is in bindings once we partition baselines based on selectivity

[eurekaka]

Development Task

Once #15281 is merged, we can call optimize() at most once for a query even if tidb_use_plan_baselines and tidb_evolve_plan_baselines are set ON, so there is no need to generate a plan first, and check if the plan is contained in the binding list anymore. This check is pretty fragile because it requires the hints generated from plan must be strictly consistent with the hints parsed from bindings.