Update MDN nav icon

[janbrasna]

One-line summary

Uses MDN logo asset instead, as referenced from other parts of the site.

Significant changes and points to review

Cleans up unused icons from long gone pages.
(Also renames some icons for consistency.)

Issue / Bugzilla link

Resolves #15864

Testing

http://localhost:8000/
(also with ./manage.py waffle_switch M24_WEBSITE_REFRESH off for og nav used by outdated locales)
http://localhost:8000/firefox/welcome/4/
http://localhost:8000/newsletter/updated/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.28%. Comparing base (9db97b8) to head (da15d11).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15865      +/-   ##
==========================================
+ Coverage   79.22%   79.28%   +0.06%     
==========================================
  Files         159      159              
  Lines        8287     8343      +56     
==========================================
+ Hits         6565     6615      +50     
- Misses       1722     1728       +6     

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