TC is a mod that always increases difficulty and is quite similar to HD.
Given we even have diffcalc/pp considerations for it it's time to move
it to the category it belongs.
This doesn't cover any other mods that might need reshuffling too
because TC is the only one that has actual impact on difficulty-based
leaderboards (as in pp) and some people are actively playing it for the
difficulty increase and not just as a fun gimmick.
After a quick search turns out it was difficulty increasing from the
start but was moved to fun in review
https://github.com/ppy/osu/pull/3569#discussion_r300085523 but I don't
really agree with that.
Also as far as I know multimods don't do anything anymore?.. I've put it
into HD multimod for consistency, but can move it to be separate if you
want
Co-authored-by: Dan Balasescu <smoogipoo@smgi.me>