Consider removing grml-zsh-config

The grml-zsh-config is installed to provide convenience functionality for zsh. However, as it exports a few environment variables (such as $EDITOR) this can lead to undesirable side effects such as FS#61945.

I'm in favor of removing it and replacing it with a minimal zsh config that only provides what we really need. Optionally some of the zsh extensions (e.g. zsh-autosuggestions, zsh-completions, zsh-history-substring-search or zsh-syntax-highlighting) can be installed.