refactor: remove error_descriptor_class and NonStaffErrorBlock

It's safe to remove this because non-staff [1] users cannot access [2]
an `ErrorBlock`. We were able to reproduce this with and without this commit
with the following results:
1. Staff users were seeing the `ErrorBlock`.
2. Non-staff users were getting an empty `<div class="vert-mod"></div>`.

In theory, error blocks should be hidden in the Learning MFE because of this
option [3]. However, when we manually set `hide_access_error_blocks` to
`False`, we kept getting identical results (with and without this commit), so
it looks that the removal `NonStaffErrorBlock` was just omitted at some point.

[1] a4ec4c1b8e/lms/djangoapps/courseware/access.py (L419-L436)
[2] a4ec4c1b8e/lms/djangoapps/courseware/access.py (L150-L151)
[3] 92ca176fde/lms/djangoapps/courseware/views/views.py (L1547-L1551)

xmodule/tests/__init__.py

@@ -29,7 +29,6 @@ from xblock.fields import Reference, ReferenceList, ReferenceValueDict, ScopeIds
 from capa.xqueue_interface import XQueueService
 from xmodule.assetstore import AssetMetadata
 from xmodule.contentstore.django import contentstore
-from xmodule.error_module import ErrorBlock
 from xmodule.mako_module import MakoDescriptorSystem
 from xmodule.modulestore import ModuleStoreEnum
 from xmodule.modulestore.draft_and_published import ModuleStoreDraftAndPublished
@@ -167,7 +166,6 @@ def get_test_system(
             'replace_urls': replace_url_service
         },
         course_id=course_id,
-        error_descriptor_class=ErrorBlock,
         descriptor_runtime=descriptor_system,
     )