phonemeset: remove support for ipa, x-sampa & cxs

This change means the only supported phonemeset is the 'features'
phonemeset. This is done for several reasons:

  1.  the phoneme model is being migrated from the old model to the
      new one;

  2.  the different phonemesets need to be handled differently for
      each class of phonemeset;

  3.  the way diacritics were supported was clunky.

Once the code has migrated to the new model, a better phonmeset
reader/writer will be implemented that takes advantage of the new
model.