Make use of the Plummer conversion constant for the time-step as well.

src/gravity/Default/gravity.h

@@ -106,7 +106,6 @@ gravity_compute_timestep_self(const struct gpart* const gp,
 
   const float epsilon = gravity_get_softening(gp, grav_props);
 
-  /* Note that 0.66666667 = 2. (from Gadget) / 3. (Plummer softening) */
   const float dt = sqrtf(2. * kernel_gravity_softening_plummer_equivalent_inv *
                          cosmo->a * grav_props->eta * epsilon * ac_inv);