Cleanup weaponorder

Moved the decls into the weapons file since they're only used there.

Removed the comment about toggling since you're not toggling the old variable but the cvar.

Made the checks symmetrical since they do the same thing - check if a cvar changed since last run. Also explicit == NULL since implicit truthiness is stupid and is known to lead to bugs. cl_weaponpriority_old is a better name because it literally contains what the cvar does AND calling it weaponorder_bypriority impied it contained weapon numbers like the other weaponorder variable, not names.

Removed unnecessary global.