feat!: Drop the legacy Tabs UI (#37557)

This change drops the legacy studio custom pages UI aka. the tab edit
page.

This work is part of https://github.com/openedx/edx-platform/issues/36108

BREAKING CHANGE: The 'legacy_studio.custom_pages' waffle flag has been
removed and the code will work as if this flag is permanently set to
False.

Co-authored-by: Kyle McCormick <kyle@axim.org>

cms/djangoapps/contentstore/rest_api/v1/serializers/course_waffle_flags.py

@@ -52,10 +52,15 @@ class CourseWaffleFlagsSerializer(serializers.Serializer):
 
     def get_use_new_custom_pages(self, obj):
         """
-        Method to get the use_new_custom_pages switch
+        Method to indicate whether or not to use the new custom pages
+
+        This used to be based on a waffle flag but the flag is being removed so we
+        default it to true for now until we can remove the need for it from the consumers
+        of this serializer and the related APIs.
+
+        See https://github.com/openedx/edx-platform/issues/37497
         """
-        course_key = self.get_course_key()
-        return toggles.use_new_custom_pages(course_key)
+        return True
 
     def get_use_new_schedule_details_page(self, obj):
         """