removed the callback function

This commit removes the synthcallback function since it wasn't doing
anything.  Also, it has been reported that this may be causing the
sluggishness when espeakup is asked to shut up.
Thanks to Chris Brannon for finding this.

espeakup.c

@@ -114,7 +114,6 @@ int main(int argc, char **argv)
 
 	/* initialize espeak */
 	espeak_Initialize(AUDIO_OUTPUT_PLAYBACK, 0, NULL, 0);
-	espeak_SetSynthCallback(SynthCallback);
 
 	/* Setup initial voice parameters */
 	set_frequency(&s, defaultFrequency, ADJ_SET);

espeakup.h

@@ -60,8 +60,6 @@ extern void queue_add_cmd(enum command_t cmd, enum adjust_t adj,
 						  int value);
 extern void queue_add_text(char *txt, size_t length);
 extern void queue_process_entry(struct synth_t *s);
-extern int SynthCallback(short *wav, int numsamples,
-						 espeak_EVENT * events);
 extern espeak_ERROR set_frequency(struct synth_t *s, int freq,
 								  enum adjust_t adj);
 extern espeak_ERROR set_pitch(struct synth_t *s, int pitch,

synth.c

@@ -28,11 +28,6 @@ const int rateMultiplier = 34;
 const int rateOffset = 84;
 const int volumeMultiplier = 22;
 
-int SynthCallback(short *wav, int numsamples, espeak_EVENT * events)
-{
-	return 0;
-}
-
 espeak_ERROR set_frequency(struct synth_t * s, int freq, enum adjust_t adj)
 {
 	espeak_ERROR rc;