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 19aac66bc262a835cae288fc3d0ccdc8e93d0fb9)

src/server/game/Entities/Unit/Unit.cpp

@@ -15257,14 +15257,9 @@ bool Unit::SetDisableGravity(bool disable)
     {
         AddUnitMovementFlag(MOVEMENTFLAG_DISABLE_GRAVITY);
         RemoveUnitMovementFlag(MOVEMENTFLAG_SWIMMING | MOVEMENTFLAG_SPLINE_ELEVATION);
-        SetFall(false);
     }
     else
-    {
         RemoveUnitMovementFlag(MOVEMENTFLAG_DISABLE_GRAVITY);
-        if (!HasUnitMovementFlag(MOVEMENTFLAG_CAN_FLY))
-            SetFall(true);
-    }
 
     static OpcodeServer const gravityOpcodeTable[2][2] =
     {
@@ -15337,14 +15332,9 @@ bool Unit::SetCanFly(bool enable)
     {
         AddUnitMovementFlag(MOVEMENTFLAG_CAN_FLY);
         RemoveUnitMovementFlag(MOVEMENTFLAG_SWIMMING | MOVEMENTFLAG_SPLINE_ELEVATION);
-        SetFall(false);
     }
     else
-    {
         RemoveUnitMovementFlag(MOVEMENTFLAG_CAN_FLY | MOVEMENTFLAG_MASK_MOVING_FLY);
-        if (!IsLevitating())
-            SetFall(true);
-    }
 
     static OpcodeServer const flyOpcodeTable[2][2] =
     {