remove hq.legacy.js

All content of hq.legacy.js is superseded (with very few modifications)
by hqwebapp/js/main.js, which is included by some super-template of all
templates that currently include hq.legacy.js.

hq.legacy.js overwrites [comes after] main.js in these cases, though, so if one of the
pages that includes it depends on the specific slightly different
functionality of the old SaveButton, that would be a problem.  I haven't
investigated or tested.

I believe this shows the differences (assuming the old SaveButton in HQ
hasn't changed much since it was copied into the formdesigner repo
(though not actually used there)):
https://github.com/mwhite/Vellum/commit/cbd6f6cfeb1ddbd5f280bad5b1c1851e3cbe75c3