Core/Grids: Replace some magic numbers with constants

(P.S.: That's why! @cs_misc.cpp)

src/server/game/Maps/MapManager.cpp

@@ -261,8 +261,8 @@ bool MapManager::ExistMapAndVMap(uint32 mapid, float x, float y)
 {
     GridCoord p = Trinity::ComputeGridCoord(x, y);
 
-    int gx=63-p.x_coord;
-    int gy=63-p.y_coord;
+    int gx = (MAX_NUMBER_OF_GRIDS - 1) - p.x_coord;
+    int gy = (MAX_NUMBER_OF_GRIDS - 1) - p.y_coord;
 
     return Map::ExistMap(mapid, gx, gy) && Map::ExistVMap(mapid, gx, gy);
 }