REST API: Plugin Delete Endpoint:  do not delete an active plugin.

The WordPress dashboard does not allow one to delete an active plugin,
neither then should the API.

+ the plugin slug could have been empty if the plugin was a single file,
and not stored in a directory

+ Instead of using the plugin id ( i.e. wordpress-seo/wp-seo ) in the request
we will use the slug ( i.e. wordpress-seo ). In browsing plugins on the .org
directory, we found that only the slug is available. I created a new function
get_plugin_id_by_slug, so that, after installation, we can convert the slug
into a plugin id, so that our newly installed plugin will pass validation

+ Removing some unneeded properties

props @roccotripaldi