chore: drop defunct skyrock and withings backends

[aviseth]

Another batch for #1176, following the same shape as #1923.

Both backends hardcode their OAuth endpoints on hosts that no longer resolve, and neither exposes a setting to point them elsewhere, so neither can complete a login for anyone:

skyrock — every endpoint is on api.skyrock.com, which has no DNS record. skyrock.com and www.skyrock.com both still resolve, so the site is alive and it is specifically the developer API that is gone.

withings — all three OAuth1 endpoints are on developer.health.nokia.com, dating from when Nokia owned Withings. Neither that host nor health.nokia.com resolves. Withings bought itself back from Nokia in 2018 and now authenticates at account.withings.com over OAuth2, so this is not a URL swap: the OAuth1 flow WithingsOAuth implements is not offered any more. Repointing would mean rewriting it as an OAuth2 backend, which felt like a separate decision for you rather than something to slip into a removal.

Removes the two modules and test_skyrock.py. withings had no test. Nothing else in the tree referenced either name.

I checked the rest of the watchlist the same way and deliberately did not include several that look dead at first glance but are not:

• osso — the demo host is dead but OSSO_BASE_URL is a setting, so self-hosted installs still work.
• monzo, mendeley — their authorization hosts (auth.getmondo.co.uk, api-oauth2.mendeley.com) are dead, but both companies and APIs are alive at api.monzo.com and api.mendeley.com. These want fixing, not removing, and I did not want to guess at the replacement endpoints without being able to test a real flow.
• etsy — api.etsy.com is gone but the backend already uses openapi.etsy.com for v3.

Happy to open the monzo/mendeley repoint as a separate PR if you would like it, and to keep working through the list in batches this size.

ruff check and ruff format --check are clean. The suite is 1178 passed / 185 skipped, with 7 failures in test_google.py::GoogleOneTapTest that are identically present on an unmodified master.

I did not add a CHANGELOG.md entry, matching #1923 — there is no unreleased section open at the moment. Say the word and I will add one.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.95%. Comparing base (099ee70) to head (e0d2261).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1939      +/-   ##
==========================================
+ Coverage   87.92%   87.95%   +0.03%     
==========================================
  Files         347      344       -3     
  Lines       13581    13544      -37     
  Branches      668      668              
==========================================
- Hits        11941    11913      -28     
+ Misses       1396     1387       -9     
  Partials      244      244              

Flag	Coverage Δ
unittests	87.95% <ø> (+0.03%)	⬆️

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