Autogenerate licence text in doc subdir from LICENCE.

Autogenerate licence text in doc subdir from LICENCE.

Now we have licence.pl, it seems to me to make very good sense to have
it generate the Halibut form(s) of the licence and copyright year as
well as the source-code forms.

As a result, I believe _no_ copies of the licence text or copyright
date exist any more except for the master one in LICENCE - so I can
completely remove the checklist section about all the places to update
it, because there's only one. Hooray!

(cherry picked from commit 774d37a0dc79441d6add265a0d360af3e53f8460)

Conflicts:
	doc/licence.but

(cherry-picker's note: the conflict was just because the deleted file
didn't have identical contents)