chore: drop defunct pocket, runkeeper, echosign, and exacttarget backends

[aviseth]

Picking up #1176 (identify and remove outdated backends). These four backends all target services that no longer expose the OAuth endpoints they hardcode, so authentication against them can't succeed anymore:

• pocket — Mozilla shut Pocket down entirely in July 2025; the apps, web app, and getpocket.com API were removed and data purged. The getpocket.com/v3/oauth endpoints are gone. (No test file existed.)
• runkeeper — ASICS discontinued the RunKeeper Health Graph API for third-party integrations, which broke the runkeeper.com/apps/authorize OAuth flow this backend depends on. (No test file existed.)
• echosign — Adobe rebranded EchoSign to Adobe Sign back in 2015 and migrated off echosign.com to adobesign.com; the hardcoded secure.echosign.com authorize/token endpoints are retired. (No test file existed.)
• exacttarget — ExactTarget was acquired by Salesforce and became Salesforce Marketing Cloud; its legacy authentication (the JWT/pre-shared-key flow this backend implements) was retired on 2019-08-01. Removed its test alongside the backend.

Each backend is referenced only in its own module (and, for exacttarget, its own test) — social_core/backends/__init__.py is empty and there's no in-repo backend registry or docs to update, so removal is complete. The full test suite passes after the removals.

I've followed the pattern of the earlier removals (#1411 itembase, and the nk / rdio / shimmering drops) and left the ### Removed CHANGELOG bullets for you to batch at release time, as with those — happy to add them here instead if you'd prefer.

I kept this to backends I'm confident are definitively dead. A few more are strong candidates but slightly more arguable (e.g. vend, now Lightspeed X-Series; angel, now Wellfound) — glad to send a follow-up if you'd like those pruned too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.92%. Comparing base (9c7b8e8) to head (f73b7b6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1923      +/-   ##
==========================================
+ Coverage   87.32%   87.92%   +0.59%     
==========================================
  Files         352      347       -5     
  Lines       13713    13581     -132     
  Branches      675      668       -7     
==========================================
- Hits        11975    11941      -34     
+ Misses       1494     1396      -98     
  Partials      244      244              

Flag	Coverage Δ
unittests	87.92% <ø> (+0.59%)	⬆️

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[nijel]

Can you please do matching edits in https://github.com/python-social-auth/social-docs/?

[aviseth]

Done — python-social-auth/social-docs#484.

Only pocket and runkeeper had pages there; echosign and exacttarget were never documented. Nothing else in the docs tree referenced either page, so it's just the two files plus their toctree entries in docs/backends/index.rst.