Remove old blog post example YAMLs

[tsaarni]

This PR removes outdated example YAML files that originated from old blog posts:

• site/examples: HTTPProxy in action blog post additional examples #1710
• site: Add Contour v1.9.0 blog post along with demo yaml files #2998

These files became orphaned after the blog section was removed from the site #6431

xref #7319

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.85%. Comparing base (180ff16) to head (71d925e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7320   +/-   ##
=======================================
  Coverage   81.85%   81.85%           
=======================================
  Files         130      130           
  Lines       15747    15747           
=======================================
  Hits        12889    12889           
  Misses       2574     2574           
  Partials      284      284           

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