[BUG] Fix knn index validation for cross cluster

[RS146BIJAY]

What is the bug?

Currently cross cluster use to depend on knn.plugin.enabled setting to validate if knn plugin is installed on nodes on not. This validation was done for all knn enabled indices. Recently knn team removed this setting as a part of 3.0.0 release. This caused this validation to break. As a part of temporary fix we disabled this validation for knn indices for CCR clusters. For permantly fixing this issue, it may require some deep dive to understand what can be the possible soln of this issue. As making any extra API call from CCR will require us to configure extra roles for leader and follower cluster in security plugin which can be a breaking change.