Fixed|libcommon: Player spawn under 3D bridge object fails

Attempting to spawn a player at a map spot below/above a 3D bridge
object failed because the dummy player mobj used to validate whether
this location was open removed the "mf2_passmobj" flag.

See: [#1033]

doomsday/plugins/common/src/p_start.cpp

@@ -868,7 +868,7 @@ boolean P_CheckSpot(coord_t x, coord_t y)
     if(!dummy) Con_Error("P_CheckSpot: Failed creating dummy mobj.");
 
     dummy->flags &= ~MF_PICKUP;
-    dummy->flags2 &= ~MF2_PASSMOBJ;
+    //dummy->flags2 &= ~MF2_PASSMOBJ;
 
     boolean result = P_CheckPosition(dummy, pos);