Remove support for deprecated "collection-default clusterprop syntax

SOLR-12827 changed the structure that Solr uses to store collection-default clusterprops all the way back in 8.0.

Prior to 8.0, collection-default clusterprops were specified using a collectionDefaults key, as in:

{
  "collectionDefaults" : {
      "numShards": 2,
      "numNrtReplicas": 2,
  }
}

Starting with 8.0, this was changed to a format that relies on additional nesting:

{
  "defaults" : {
    "collection": {
      "numShards": 2,
      "numNrtReplicas": 2,
    }
  }
}

SOLR-12827 deprecated the old format and removed all documentation referencing it. The code itself though still has a few vestiges of this old format. This ticket aims to remove these last traces of the old format.