Remove restart-required and %postun logic
    
    The current effect of the restart-required logic is that we only
    execute %postun of the old package on upgrade and not %posttrans of
    the new package. This is exactly what we want to avoid as deploying
    fixes to %postun requires two full rollouts before they take effect
    compared to %posttrans which takes effect immediately.
    
    Because we've already done a release with the restart-required logic,
    the net effect on upgrade/downgrade now will be the following:
    
    - On upgrade, the restart-required file will not be created anymore,
    which means that %postun of the old package will be skipped. %posttrans
    of the new package will run.
    - On downgrade, the restart-required file will be created, which means
    the %posttrans of the old package will run. As a result, we don't need
    the %postun scriptlets anymore in the new package and can remove the logic
    and solely rely on %posttrans in the future.