phonemeset: remove the English-based phonemesets

These phonemeset suffer from several problems:

1. they don't really fit the conventions of IPA-style
   transcriptions (e.g. Arpabet);

2. the affricates, diphthongs and triphthongs are incorrectly
   modelled (the underlying phonetic model does not properly
   support them);

3. they mix phonemes and phoneme mapping.

Removing these makes it easier to fix the underlying phonetic model
incrementally, after which support for them can be added back in
the correct way.
phoneme: remove the afr, dpf and dpr features

These features are used to describe affricates and diphthongs.
However, this representation is wrong as it restricts the number
of these that can be represented and provides an incorrect
description of the phonemes.

The correct way to model these is to use a pair of phonemes that
describe the start and end positions of the phonemes and mark the
second phoneme as either:

1.  tied (using a tie-bar) for affricates, diphthongs or
    triphthongs; or

2.  non-syllabic for diphthongs or triphthongs.