Core/Movement: enable item use and spell cast when dismounting

Remove error message and allow spell cast or item use from
mounted state on ground, especially when sitting on
a landed flying mount. Current behavior is that the player
will get the error message "You can't do that while moving"
when trying to cast a spell or pick up an item
while sitting on a flying mount after having landed.

By joschiwald, closes #6236

(cherry picked from commit 19aac66bc2)

src/server/game/Movement/MotionMaster.cpp

@@ -473,8 +473,11 @@ void MotionMaster::MoveFall(uint32 id /*=0*/)
     if (std::fabs(_owner->GetPositionZ() - tz) < 0.1f)
         return;
 
+    _owner->SetFall(true);
+
+    // don't run spline movement for players
     if (_owner->GetTypeId() == TYPEID_PLAYER)
-        _owner->SetFall(true);
+        return;
 
     Movement::MoveSplineInit init(_owner);
     init.MoveTo(_owner->GetPositionX(), _owner->GetPositionY(), tz, false);