Delete unused classification_groupings page (duplicate of classifications page)

🤖 Written by Claude

`/classification/groupings` (url name `classification_groupings`, view `classification/views/views.py:282`, template `classification/templates/classification/classification_groupings.html`) is not reachable from the UI.

The only references in the codebase are:
* `classification/urls.py:96` — the route
* `classification/tests/utils/test_urls.py:111` — a URL test asserting 200

No menu item and no `{% url 'classification_groupings' %}` anywhere. (The `{% classification_groupings %}` usages in other templates are the inclusion template tag `classification/tags/classification_groupings.html`, which is a different thing.)

### Background

Both `classifications.html` and `classification_groupings.html` came in with the grouping merge (a731a7d6f, Feb 2025), where `classifications.html` took over pointing at `classification_grouping_datatables`. `classification_groupings.html` looks like the prototype that never got removed, and the two have been drifting since:

| | `classifications.html` (live) | `classification_groupings.html` |
| --- | --- | --- |
| Search and classify form | yes | no |
| ID filter input | yes | JS reads `#id_filter`, no such element in the page |
| User filter form | yes | no |
| Superuser import / ImportedAlleleInfo status panel | yes | no |
| External lab toggle | added in #394 follow-up | had it since d70a6eb25 |

Fixes have been landing on both by hand - the #1629 datatable race condition fix was duplicated into each. The external lab filter for #394 was added only to the dead page, so `CLASSIFICATION_GRID_EXTERNAL_LAB_FILTER = True` appeared to do nothing on the real Classifications page until the toggle was added to `classifications.html`.

### Proposed

Delete the dead page:
* `classification/templates/classification/classification_groupings.html`
* `classification_groupings` view in `classification/views/views.py`
* the route in `classification/urls.py`
* the entry in `classification/tests/utils/test_urls.py`

Worth a quick check first that nobody has the URL bookmarked / that it isn't linked from anything outside this repo.