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

@@ -60,7 +60,6 @@ int wavefile_ix; // a wavefile to play along with the synthesis
 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)
 {