mbrola: remove the current code

This is going to be rewritten with a better API and implementation.
mbrola: use fork & execlp to start an mbrola process
mbrola: set-up pipes for stdin, stdout and stderr
mbrola: read the sample rate from the WAV header
mbrola: switch to non-blocking I/O
mbrola: hook up audio output
mbrola: hook up prosody writer functionality
mbrola: don't wait for output if mbrola is idle
pulse: drain the audio before closing

If close() is called while pulseaudio is still playing audio, it
would cut off the remaining audio. By calling |pa_simple_drain|
before |pa_simple_free|, any remaining audio will be played
correctly.