Use favicon from gitlab.haskell.org

The current .png is presumably the wrong shape and looks flattened.

Plus, mkdocs complained that 'site_favicon' was an unrecognised
configuration name, so I used the method described in the docs instead:

https://www.mkdocs.org/user-guide/customizing-your-theme/#using-the-theme_dir

mkdocs.yml

@@ -2,7 +2,6 @@ site_name: GHCup
 site_url: https://www.haskell.org/ghcup
 site_description: GHCup is the main installer for the general purpose language Haskell.
 site_author: GHCup Team
-site_favicon: haskell_logo.png
 
 repo_url: https://github.com/haskell/ghcup-hs