[cleanup] Remove deprecated load config from file

[Wauplin]

This PR removes the possibility to load a Config using a file URL directly. This is a deprecated feature expected to be removed in transformers v5:

    warnings.warn(
        f"Using `from_pretrained` with the url of a file (here {url}) is deprecated and won't be possible anymore in"
        " v5 of Transformers. You should host your file on the Hub (hf.co) instead and use the repository ID. Note"
        " that this is not compatible with the caching system (your file will be downloaded at each execution) or"
        " multiple processes (each process will download the file in a different temporary file).",
        FutureWarning,
    )

I've also removed the legacy test test_legacy_load_from_one_file testing this feature.

---

In addition, this allowed me to cleanup the is_remote_url and download_url helpers and to entirely avoid importing http_get from huggingface_hub (which is to be considered a "semi-private" helper. I have refactored a few tests using it to default to hf_hub_download instead.

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: metaclip_2

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Wauplin]

This PR is ready for review. CI is failing on

FAILED tests/models/kosmos2/test_modeling_kosmos2.py::Kosmos2ModelTest::test_assisted_decoding_matches_greedy_search_1_same - AssertionError: False is not true

which looks unrelated to the PR. I already retriggered the CI once but it didn't help.

[LysandreJik]

Really satisfying PR, thanks for handling this @Wauplin!