SmrPlanet.class.php: remove duplicated code
SmrPlanet.class.php: remove duplicated code

Refactor the common logic from moveMountedWeapon{Up,Down} into a
new private function swapMountedWeapons.

smrealms/smr,hemberger/smr,smrealms/smr,hemberger/smr,hemberger/smr,hemberger/smr,smrealms/smr