Refactor: convert "Clear Useless Actions" operator to current Action API

Convert the code for the "Clear Useless Actions" 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/anim.py

@@ -352,6 +352,16 @@ class ClearUselessActions(Operator):
     def poll(cls, _context):
         return bool(bpy.data.actions)
 
+    @staticmethod
+    def has_fcurves(action: bpy.types.Action) -> bool:
+        for layer in action.layers:
+            for strip in layer.strips:
+                assert strip.type == 'KEYFRAME'
+                for channelbag in strip.channelbags:
+                    if channelbag.fcurves:
+                        return True
+        return False
+
     def execute(self, _context):
         removed = 0
 
@@ -365,7 +375,7 @@ class ClearUselessActions(Operator):
                 # if it has F-Curves, then it's a "action library"
                 # (i.e. walk, wave, jump, etc.)
                 # and should be left alone as that's what fake users are for!
-                if not action.fcurves:
+                if not self.has_fcurves(action):
                     # mark action for deletion
                     action.user_clear()
                     removed += 1