STB Vorbis: eliminate inaccessible branch

The eliminated code removes the (ch == 1) branch, which is scoped
within this if condition: `if (rtype == 2 && ch != 1)`, therefore
the (ch == 1) branch will never be taken.

Fixes #842.

stb_vorbis.c

@@ -2130,46 +2130,6 @@ static void decode_residue(vorb *f, float *residue_buffers[], int ch, int n, int
                ++class_set;
                #endif
             }
-         } else if (ch == 1) {
-            while (pcount < part_read) {
-               int z = r->begin + pcount*r->part_size;
-               int c_inter = 0, p_inter = z;
-               if (pass == 0) {
-                  Codebook *c = f->codebooks+r->classbook;
-                  int q;
-                  DECODE(q,f,c);
-                  if (q == EOP) goto done;
-                  #ifndef STB_VORBIS_DIVIDES_IN_RESIDUE
-                  part_classdata[0][class_set] = r->classdata[q];
-                  #else
-                  for (i=classwords-1; i >= 0; --i) {
-                     classifications[0][i+pcount] = q % r->classifications;
-                     q /= r->classifications;
-                  }
-                  #endif
-               }
-               for (i=0; i < classwords && pcount < part_read; ++i, ++pcount) {
-                  int z = r->begin + pcount*r->part_size;
-                  #ifndef STB_VORBIS_DIVIDES_IN_RESIDUE
-                  int c = part_classdata[0][class_set][i];
-                  #else
-                  int c = classifications[0][pcount];
-                  #endif
-                  int b = r->residue_books[c][pass];
-                  if (b >= 0) {
-                     Codebook *book = f->codebooks + b;
-                     if (!codebook_decode_deinterleave_repeat(f, book, residue_buffers, ch, &c_inter, &p_inter, n, r->part_size))
-                        goto done;
-                  } else {
-                     z += r->part_size;
-                     c_inter = 0;
-                     p_inter = z;
-                  }
-               }
-               #ifndef STB_VORBIS_DIVIDES_IN_RESIDUE
-               ++class_set;
-               #endif
-            }
          } else if (ch > 2) {
             while (pcount < part_read) {
                int z = r->begin + pcount*r->part_size;