remove outdated TODO.txt file

[aaronayres35]

This PR simple removes a TODO.txt file that has not been touched since 2011. See comment here: #173 (comment)

I will gladly open github issues for these if they are still relevant.
The first bullet seems like it may still be relevant:

apptools/apptools/persistence/state_pickler.py

Lines 1004 to 1009 in af64aaf

	if 'tvtk_classes' in mod_name:
	# FIXME: This sort of special-case is probably indicative of something
	# that needs more thought, plus it makes it tought to decide whether
	# this component depends on tvtk!
	from tvtk.api import tvtk
	return getattr(tvtk, class_name)()

The second seems to be unrelated to apptools. Also apptools.help we know needs tests, and with envisage 5.0.0 about to release the last bullet about envisage2 seems to no longer be relevant