Refactor: convert sequencer "Clear Fades" to current Action API

Convert the code for the sequencer "Clear Fades" operator from the
legacy Actions API to the current API.

No functional changes.

This is part of #146586

Pull Request: https://projects.blender.org/blender/blender/pulls/147060

scripts/startup/bl_operators/sequencer.py

@@ -14,6 +14,7 @@ from bpy.props import (
     IntProperty,
 )
 from bpy.app.translations import pgettext_rpt as rpt_
+from bpy_extras import anim_utils
 
 
 def _animated_properties_get(strip):
@@ -149,10 +150,10 @@ class SequencerFadesClear(Operator):
         animation_data = scene.animation_data
         if animation_data is None:
             return {'CANCELLED'}
-        action = animation_data.action
-        if action is None:
+        channelbag = anim_utils.action_get_channelbag_for_slot(animation_data.action, animation_data.action_slot)
+        if channelbag is None:
             return {'CANCELLED'}
-        fcurves = action.fcurves
+        fcurves = channelbag.fcurves
         fcurve_map = {
             curve.data_path: curve
             for curve in fcurves