Remove Util.pcequiv() and stop pretending Java colls are equiv to Clojure colls

The Util.pcequiv() method

tries to get equiv semantics (cross-class number equality) for cases of mixed Clojure/Java collection comparison. However, this is not a sustainable direction and we would like to stop doing this.

Attached patch removes this and changes calling code to only call equiv when both collections are IPersistentCollection.