chore(lightning): remove Blink and LNbits backends

[callebtc]

Close #1109

Summary

• remove the Blink and LNbits Lightning backend implementations and exports
• remove their settings, environment examples, CI/regtest configuration, and documentation references
• remove dedicated and mocked tests for both backends

Validation

• make check
• pytest -q tests/lightning/test_lightning_backends_mocked.py (28 passed)
• pytest -q tests/mint/test_mint_melt.py -k early_return (2 passed, 22 deselected)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.08%. Comparing base (cf1cf76) to head (a805d82).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1110      +/-   ##
==========================================
+ Coverage   74.06%   74.08%   +0.01%     
==========================================
  Files         116      114       -2     
  Lines       13369    12962     -407     
==========================================
- Hits         9902     9603     -299     
+ Misses       3467     3359     -108     

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[KvngMikey]

lgtm