Stop supporting '[this]' as a comment reference.

The analyzer does not support it and does not intend to, so this change aligns
with that effort. It makes analyzing the gap between analyzer and dartdoc
easier as well.

Fixes dart-lang#3761

Author: srawlins

lib/src/model/container.dart

@@ -242,7 +242,6 @@ abstract class Container extends ModelElement
             .addEntriesIfAbsent(modelElement.parameters.generateEntries());
       }
     }
-    referenceChildren['this'] = this;
     return referenceChildren;
   }();
 

test/end2end/model_test.dart

@@ -2805,9 +2805,6 @@ void main() async {
                 doAwesomeStuff, 'BaseForDocComments.aNonDefaultConstructor'),
             equals(MatchingLinkResult(aNonDefaultConstructor)));
 
-        expect(referenceLookup(doAwesomeStuff, 'this'),
-            equals(MatchingLinkResult(baseForDocComments)));
-
         expect(referenceLookup(doAwesomeStuff, 'value'),
             equals(MatchingLinkResult(doAwesomeStuffParam)));