fix: drop stale obs/updates in rfced sync

[jennifer-richards]

Removes updates and obs relationships that are not present in the RFC Editor index when syncing.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.36%. Comparing base (1799245) to head (986e2f5).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/sync/rfceditor.py	60.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10543      +/-   ##
==========================================
- Coverage   88.39%   88.36%   -0.03%     
==========================================
  Files         326      327       +1     
  Lines       43813    44092     +279     
==========================================
+ Hits        38730    38964     +234     
- Misses       5083     5128      +45     

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