revert: fix(a): prevent Opera from incorrectly navigating on link click

This reverts commit c81d8176cc.

This commit causes several issues (#1651, #1674, #1662) and doesn't even
contain a test that proves that anything on Opera got actually fixed.

If the original Opera resurfaces, we'll fix it properly.

src/ng/directive/a.js

@@ -27,7 +27,6 @@ var htmlAnchorDirective = valueFn({
         // if we have no href url, then don't navigate anywhere.
         if (!element.attr('href')) {
           event.preventDefault();
-          return false; // Needed for opera
         }
       });
     }