fix(seo): remove contributors.txt from robots.txt (#11311)

We added */contributors.txt to our robots.txt with the intention 
of hiding those pages from Google, but it seems to prevent Google 
from seeing the X-Robots-Tag header, keeping the pages in the index,
although we would like to see them removed from the index.

tool/build-robots-txt.ts

@@ -12,7 +12,6 @@ User-agent: *
 Sitemap: https://developer.mozilla.org/sitemap.xml
 
 Disallow: /api/
-Disallow: /*/contributors.txt
 Disallow: /*/files/
 Disallow: /media
 `;