fix($rootScope): do not mark $watchCollectionInterceptor as $stateful

This interceptor does not store any state, it only reads the (complex)
state of its input.

Removing the `$stateful` flag will allow some expressions (those with
`inputs` such as object/array literals) to avoid invoking the
`$watchCollectionInterceptor` and avoid creating the literals on each
digest.

Author: jbedard

src/ng/rootScope.js

@@ -581,8 +581,6 @@ function $RootScopeProvider() {
        *    de-registration function is executed, the internal watch operation is terminated.
        */
       $watchCollection: function(obj, listener) {
-        $watchCollectionInterceptor.$stateful = true;
-
         var self = this;
         // the current value, updated on each dirty-check run
         var newValue;