tectonic/dpx-pdfobj: remove useless assignation

k is reassigned on the next iteration of the loop. Hence, the k++ is
useless.

Reported by @Mrmaxmeier

Author: ronnychevalier

tectonic/dpx-pdfobj.c

@@ -1694,7 +1694,7 @@ apply_filter_TIFF2_1_2_4 (unsigned char *raster,
             }
         }
         if (outbits > 0) {
-            raster[k] = (outbuf << (8 - outbits)); k++;
+            raster[k] = (outbuf << (8 - outbits));
         }
     }
     free(prev);