mbrola: restrict the phones to voice phones only

This removes the compatiblity/alias phones from the voice files.
There are several issues using compatibility phones that have not
been fully resolved:

  1.  They need to be specified in all voices that don't support
      the phoneme.

  2.  The actual phone used depends on the supported diphones.

  3.  The search space for fallback phones is well defined, but
      depends on the required language/accent.

  4.  Different features of the voice may trigger different
      features in the language/accent; for example, a rhotic
      voice will work differently to a non-rhotic voice when
      speaking English.