Revert "Use rigid advance"

This reverts commit 5421ff4.

Examples/Physics_applications/free_electron_laser/inputs_test_1d_fel

@@ -62,6 +62,7 @@ electrons.uz = gamma_bunch
 electrons.zmax = -25e-6
 electrons.zmin = -125e-6
 electrons.zinject_plane=0.0
+electrons.rigid_advance=0
 
 positrons.charge = q_e
 positrons.injection_style = nuniformpercell
@@ -76,6 +77,7 @@ positrons.uz = gamma_bunch
 positrons.zmax = -25e-6
 positrons.zmin = -125e-6
 positrons.zinject_plane=0.0
+positrons.rigid_advance=0
 
 warpx.do_moving_window = 1
 warpx.moving_window_dir = z