Remove 'Structure' preferred class from elephants, shorten max range.

Elephants were designed to prefer targeting structures. This is not particularly historically accurate, and makes them rather annoying to use against enemy units, reducing their efficiency.

This simply removes the preferred class, making elephants easier to handle while keeping their usage as siege units for some civilisations.

Max-range is reduced slightly as the previous one was extremely large for a melee unit.

Approved by: Nescio, wraitii

Patch by: borg-

Differential Revision: https://code.wildfiregames.com/D2786