DH_Vehicles package - clean up unused & clashing variables

[Matt Hands]

Removing sources of non-critical warnings when compiling DH_Vehicles:

DH_BoatVehicle - remove unused class variables, some of which conflict with function parameters or local variables (removed MaxPitchSpeed, BoatFloatTraceDistance, HitDist, HoverAdd, TraceStart, TraceEnd, HitLocation, HitNormal, HitActor, bBoatFloat)

DH_HigginsBoat - removed deprecated default property for MaxPitchSpeed

DH_MobileResupplyAttachment - removed unused local variables bEnemyGrenadeFound & bEnemySmokeFound in function Timer

DH_ROWheeledVehicle - removed unused local variable Pawn P in function ServerStartEngine

[Matt Hands]

Fixed in commit 29bcdd1