Remove undocumented "continued" modifier

This class was only ever used by one site, and it's completely dependent
on a fixed pixel font size of the h2.  If it's needed in specific
contexts, we'll add it in those contexts.
Remove mobile transitions

Mobile is about to change pretty dramatically.  What we have now will
only be used as a no-js fallback.  So it doesn't make much sense to
maintain a fairly complex, hacky max-height transition.