Remove nested function eval in blockHelperMissing

This only worked when a function returned a function, which was then evaluated. This seems like unexpected behavior so this was dropped.

lib/handlebars/base.js

@@ -69,8 +69,6 @@ function registerDefaultHelpers(instance) {
     var inverse = options.inverse,
         fn = options.fn;
 
-    if (isFunction(context)) { context = context.call(this); }
-
     if(context === true) {
       return fn(this);
     } else if(context === false || context == null) {