burn-normalize: disable normalisation for now

Since the port to GStreamer 1.0 the rganalysis pipleline stops after the
first track is processed and pauses the entire audio disc burning process
indefinitely.

Disable the normalisation plugin for now as I haven't yet been able to
determine an appropriate fix for this issue.

https://bugzilla.gnome.org/show_bug.cgi?id=699599