* Remove ERT and "package" installation in CI
Since Emacs 23 is dropped for quite some time now, remove installation
of ERT and "package", because they are built-in since Emacs 24.
* Remove "prereqs" function left-overs
Currently, there are no builds for Emacs 28.
Emacs 28 seems have introduced warnings about the line length in code
documentation. These warnings have been fixed as well.