Comment out a dead increment.

Found by Clang's static analyzer.
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 4b0fd91..661bf19 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -642,7 +642,7 @@
             writer->buffer, writer->pos,
             digits, d_pos, spec->n_remainder);
         writer->pos += spec->n_remainder;
-        d_pos += spec->n_remainder;
+        /* d_pos += spec->n_remainder; */
     }
 
     if (spec->n_rpadding) {