removed dx_max from space.h since it's not needed anymore

grep tells me it indeed isn't needed anywhere any longer, and I've been able to successfully run a few dozen steps for SmallCosmoVolume_hydro.

src/space.h

@@ -112,9 +112,6 @@ struct space {
   /*! The minimum top-level cell width allowed. */
   double cell_min;
 
-  /*! Current maximum displacement for particles. */
-  float dx_max;
-
   /*! Space dimensions in number of top-cells. */
   int cdim[3];