ref(grouping): remove unused grouping-configs endpoint from backend

[shayna-ch]

Remove unused grouping-configs endpoint from backend. The GroupingConfigsEndpoint class isn't getting deleted because it is still being used in the non-scoped grouping configs endpoint.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
26818	1	26817	601

View the top 1 failed test(s) by shortest run time

tests.sentry.api.endpoints.test_grouping_configs.GroupingConfigsWithSlugTest::test_with_slug

Stack Traces | 1.72s run time

#x1B[1m#x1B[.../api/endpoints/test_grouping_configs.py#x1B[0m:33: in test_with_slug
    resp = self.get_response(self.organization.slug)
#x1B[1m#x1B[.../sentry/testutils/cases.py#x1B[0m:576: in get_response
    else reverse(self.endpoint, args=args)
#x1B[1m#x1B[31m.venv/lib/python3.13.../django/urls/base.py#x1B[0m:98: in reverse
    resolved_url = resolver._reverse_with_prefix(view, prefix, *args, **kwargs)
#x1B[1m#x1B[31m.venv/lib/python3.13.../django/urls/resolvers.py#x1B[0m:831: in _reverse_with_prefix
    raise NoReverseMatch(msg)
#x1B[1m#x1B[31mE   django.urls.exceptions.NoReverseMatch: Reverse for 'sentry-api-0-organization-grouping-configs' not found. 'sentry-api-0-organization-grouping-configs' is not a valid view function or pattern name.#x1B[0m

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[lobsterkatie]

Two quick suggestions on this one:

• It'd be helpful to mention in the PR description why the GroupingConfigsEndpoint class isn't getting deleted alongside the route (it's still used in the non-org-scoped grouping configs endpoint).
• Given that it's still used in that other route, I haven't looked at the code, but you might be able to fix the failing test by just switching to using its route instead. In the long run, I suspect that endpoint may also go, so whatever the quickest solution to getting it working is, I'd go with that.

Also, the tiniest of nits: The endpoint was used in the grouping info section, but its original purpose was for more than that, so I'd just make it ref(grouping).