Revert "toolhead: Use dict for step generation flush times. (#6303)"

This reverts commit 6749985302.

A defect was found in the above commit (the input shaper code calls
note_step_generateion_scan_time() for many steppers, so the
input_shaper class can't be used as the index).

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/kinematics/extruder.py

@@ -70,11 +70,15 @@ class ExtruderStepper:
         self.stepper.set_trapq(extruder.get_trapq())
         self.motion_queue = extruder_name
     def _set_pressure_advance(self, pressure_advance, smooth_time):
+        old_smooth_time = self.pressure_advance_smooth_time
+        if not self.pressure_advance:
+            old_smooth_time = 0.
         new_smooth_time = smooth_time
         if not pressure_advance:
             new_smooth_time = 0.
         toolhead = self.printer.lookup_object("toolhead")
-        toolhead.note_step_generation_scan_time(self, new_smooth_time * .5)
+        toolhead.note_step_generation_scan_time(new_smooth_time * .5,
+                                                old_delay=old_smooth_time * .5)
         ffi_main, ffi_lib = chelper.get_ffi()
         espa = ffi_lib.extruder_set_pressure_advance
         espa(self.sk_extruder, pressure_advance, new_smooth_time)