Revert "Don't include codepoint 0 in the results of collect_unicodes."

This reverts commit 14ad96ffbf77c33d8d33d2686d17c2375381989e.

This was wrong.  My bad!

https://github.com/harfbuzz/harfbuzz/issues/2031
diff --git a/src/hb-ot-cmap-table.hh b/src/hb-ot-cmap-table.hh
index 37d3378..e664615 100644
--- a/src/hb-ot-cmap-table.hh
+++ b/src/hb-ot-cmap-table.hh
@@ -1324,13 +1324,7 @@
     }
 
     void collect_unicodes (hb_set_t *out) const
-    {
-      subtable->collect_unicodes (out);
-      // Never include unicode codepoint 0 in the set.
-      // This is assumed to be the notdef glyph.
-      out->del (0);
-    }
-
+    { subtable->collect_unicodes (out); }
     void collect_variation_selectors (hb_set_t *out) const
     { subtable_uvs->collect_variation_selectors (out); }
     void collect_variation_unicodes (hb_codepoint_t variation_selector,