remove reference to entities.tcl

This script was ported to Python in Graphviz commit
b42317e5029d326ba8ca7c2387b9ecda31471427. But more importantly, it is unclear
what this text is describing. The TCL script generated a C header. What follows
here is a PNG image. This seems to have been true even when this content was
initially added (75f20e47), so I do not see how
the TCL script can have been responsible for generating this image.