[Cleanup] Remove deprecated plugins.neural_search.hybrid_search_disabled setting

Setting plugins.neural_search.hybrid_search_disabled is a legacy feature flag that was introduced in the early days of hybrid query development as a temporary workaround for concurrent search conflicts. The original code comment explicitly states:

Gates the functionality of hybrid search. Currently query phase searcher added with hybrid search will conflict with concurrent search in core. Once that problem is resolved this feature flag can be removed.

Concurrent search conflicts have been resolved, and hybrid search is now a stable feature. This setting is no longer needed and should be removed to reduce code complexity and configuration surface area.

Documentation (follow-up): Remove "Disabling hybrid queries" section from hybrid query documentation https://docs.opensearch.org/latest/query-dsl/compound/hybrid/