code cleanup: remove unused int vowel_transition[4]

There's two variables with the same name:
1) in struct PHONEME_DATA, used by *phdata, *phdata_next and *phdata_prev
2) an unnecessary one that's only being memset() twice but never used
   for anything

src/libespeak-ng/synthdata.c

 int wavefile_amp;
 
 int seq_len_adjust;
-int vowel_transition[4];
 
 static espeak_ng_STATUS ReadPhFile(void **ptr, const char *fname, int *size, espeak_ng_ERROR_CONTEXT *context)
 {

src/libespeak-ng/synthesize.c

 		syllable_end = wcmdq_tail;
 		SmoothSpect();
 		syllable_centre = -1;
-		memset(vowel_transition, 0, sizeof(vowel_transition));
 	}
 }
 
 		syllable_end = wcmdq_tail;
 		syllable_centre = -1;
 		last_pitch_cmd = -1;
-		memset(vowel_transition, 0, sizeof(vowel_transition));
 		memset(&worddata, 0, sizeof(worddata));
 		DoPause(0, 0); // isolate from the previous clause
 	}

src/libespeak-ng/synthesize.h

 
 extern int wavefile_ix;
 extern int wavefile_amp;
-extern int vowel_transition[4];
 
 #define N_ECHO_BUF 5500   // max of 250mS at 22050 Hz
 extern int echo_head;