clean-up: remove one leftover of Ref::THIS (#996)

for the record: while it is indeed sweet-and-simple to support Ref::THIS
here, it is near impossible to represent it in general, in a setup with
multiple "onion-layers". The reason is, we'd have to incorporate such
special treatment into the /selector predicate/, which in turn undermines
the ability to pick the right onion layer to handle a given diff verb,
since "Ref::THIS" is a generic marker and we have no other data to base
the decision in the selector on.

src/lib/diff/tree-mutator-collection-binding.hpp

@@ -154,8 +154,7 @@ namespace diff{
         locate (GenNode const& targetSpec)
           {
             if (not collection.empty()
-                and (Ref::THIS.matches(targetSpec.idi)
-                     or matches (targetSpec, collection.back())))
+                and matches (targetSpec, collection.back()))
               return lastElm();
             else
               return search (targetSpec, eachElm(collection));