Fix PreferredClasses following rP23831

rP23831 introduced a "Unit+!Ship" preferred class. However, Preferred classes don't actually use MatchesClassList, and thus this resulted in no preference whatsoever. This is fixed.

Further, Macemen & elephants have no particular preferred classes, unlike all other land units which are either Unit+!Ship or Human (which implies !Ship). This is fixed by setting them to !Ship, for consistency.

Reported by: snelius (and also Valirhant, I believe)

Trac Tickets: #5945

Differential Revision: https://code.wildfiregames.com/D3442