Remove hill giants

Remove hill giants

There's no real need to have an "introductory" giant, and their vanilla
status dilutes the rest of the giants. Cyclops and ettins work better
as an early set of giants (compare mid fire/frost/stone and then late
titan/juggernaut/iron).

Vault surgery has been performed, with two-headed ogre for the most part
taking their place. 2HO are being buffed in oncoming commits, and threat
level between hill giant / 2HO should remain more or less the same for
these affected vaults.