docs: Drop default branch section from README

It's been more than 3 years since we renamed the default branch,
people have probably got the message by now.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/352>

README.md

@@ -80,19 +80,6 @@ GSettings key.
 
      Adds a simple workspace switcher to the top bar.
 
-## Default branch
-
-The default development branch is `main`. If you still have a local
-checkout under the old name, use:
-```sh
-git checkout master
-git branch -m master main
-git fetch
-git branch --unset-upstream
-git branch -u origin/master
-git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main
-```
-
 ## License
 
 GNOME Shell Extensions are distributed under the terms of the GNU General