Remove combined-manifesto switch, and take old Leadership page out of circulation

[stephaniehobson]

One-line summary

Clean up from WT-1320

Significant changes and points to review

• Remove combined-manifesto switch, content is now permanent
• Remove un-used old leadership page from circulation
  • leave models for follow up removal.

Note: The work to remove stub-attribution from non-Firefox pages on bedrock was bundled with this accidentally.

Issue / Bugzilla link

WT-1389

Testing

http://localhost:8000/en-US/about/
http://localhost:8000/en-US/about/manifesto/
http://localhost:8000/en-US/foundation/moco/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.80%. Comparing base (ac7c3a9) to head (6c36a4b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17253      +/-   ##
==========================================
- Coverage   82.84%   82.80%   -0.05%     
==========================================
  Files         178      178              
  Lines        9409     9409              
==========================================
- Hits         7795     7791       -4     
- Misses       1614     1618       +4     

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[stevejalim]

I don't think this will stop is deleting the existing LeadershipPage if it still exists. Worst-case we can temporarily put it back in before we drop the actual model

[stevejalim]

LGTM - one small thought

r+wc