Removing remaining instances of multiple_clusters

The following discussion from !35094 (merged) should be addressed:

• @tigerwnz started a discussion: (+6 comments)

  Since this will now be true in 100% of cases we should be able to remove it completely, along with any view code that used false. It looks like we check this in three places, when we create GCP and User clusters, and also when editing a cluster.

  Interestingly I couldn't find a check in the Vue code for EKS clusters. Perhaps this was an existing bug where we didn't show a different view for FOSS?