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)

setup.py

@@ -24,7 +24,6 @@ XBLOCKS = [
     "library_content = xmodule.library_content_module:LibraryContentBlock",
     "library_sourced = xmodule.library_sourced_block:LibrarySourcedBlock",
     "lti = xmodule.lti_module:LTIBlock",
-    "nonstaff_error = xmodule.error_module:NonStaffErrorBlock",
     "poll_question = xmodule.poll_module:PollBlock",
     "problem = xmodule.capa_module:ProblemBlock",
     "randomize = xmodule.randomize_module:RandomizeBlock",