fixup! Mirror master and main

For an unspecified transition phase, we installed a GitHub workflow to
keep updating between the `main` and the `master` branch, with `main`
being the _actual_ main branch.

This was four months ago. It's time to end the transition phase, remove
this GitHub workflow and the `master` branch.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho