Stop mirroring the main branch to the master branch

In preparation for removing the `master` branch from
https://github.com/git-for-windows/git-for-windows.github.io,
let's remove the GitHub workflow that would _immediately_
reinstate it.

The pages are already built off of the `main` branch, so we're
good to go.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho