Remove barrett_diagram file duplicated by tree.html

* Makefile.direct (DOC_FILES): Remove doc/barrett_diagram entry.
* Makefile.dj (OTHER_FILES): Likewise.
* doc/doc.am (dist_pkgdata_DATA): Likewise.
* doc/barrett_diagram: Remove file (as the content is duplicated in
doc/tree.html).
Remove non-existing DISCARD_WORDS from GC data structure ASCII diagram

* doc/tree.html (picture): Remove DISCARD_WORDS (since always zero
and already removed from the code in gc-6.0alpha6).