Tag reference doesn't need to be exclusive

This makes the implementation more complex and the
feature is of marginal benefit, possibly detrimental.
(What if I want to compare two tags and figure out
which one is more appropriate?)

js/id/ui/preset.js

@@ -85,13 +85,7 @@ iD.ui.preset = function(context, entity, preset) {
         d3.event.stopPropagation();
         d3.event.preventDefault();
 
-        _.forEach(shown(), function(other) {
-            if (other.id === field.id) {
-                other.reference.toggle();
-            } else {
-                other.reference.hide();
-            }
-        });
+        field.reference.toggle();
 
         render();
     }

js/id/ui/taglist.js

@@ -102,13 +102,7 @@ iD.ui.Taglist = function(context, entity) {
 
         row.call(iD.ui.TagReferenceButton()
             .on('click', function(tag) {
-                tags.forEach(function(other) {
-                    if (other.key === tag.key) {
-                        other.reference.toggle();
-                    } else {
-                        other.reference.hide();
-                    }
-                });
+                tag.reference.toggle();
             }));
 
         row.each(function(tag) {