BUG-614: Remove YangtoolsMappingHelper in favor of BindingReflections

As it turns out YangtoolsMappingHelper is an old duplicate of
BindingReflections, with a single isNotification() method. That method
is buggy, as it does not perform all the checks required. Remove it and
use the unified method exposed from BindingReflections.

Change-Id: Ifc2ef9746c2556feac05466439ed910208289413
Signed-off-by: Robert Varga <rovarga@cisco.com>