Removed plugins are not removed from disc

[ssbarnea]

Removed plugins are not removed from disc when running antsibull-docs collection-plugins and that is a problematic because it leads to bugs: when someone removes or renames a plugin, they might forget to update the documentation.

As other docs might be inside the same folder, we should check of the "Generated by" marker before removing the files.

[felixfontein]

This is not a bug, but intended design. If you want files to be deleted, you have to do that yourself. For example you can create a wrapper script, see the script created by antsibull-docs sphinx-init.