ClassIcons tweaks/fixes

ClassIcons stopped working properly because when the player changed spec as a Warlock or a Priest, the element unregistered PLAYER_TALENT_UPDATE/SPELLS_CHANGED, so when the player changed specs back the element wouldn't show the icons.

This PR fixes that, removes some old code like the DARK_FORCE power type that does not exist any more (or atleast is never used for anything), plus it cleans up the codebase after the unnecessary checks have been changed/removed.