hook_uninstall deletes file usage

[indigoxela]

I know, this has been inherited from Drupal, but in Backdrop CMS this uninstall handling has a bad side effect.

https://github.com/backdrop-contrib/imce/blob/1.x-1.x/imce.install#L20

It's one of the causes for the problem reported here: backdrop/backdrop-issues#3857

Instead of just deleting the usage, leaving all inline files in an odd state (permanent, no file usage record), something different should be done on uninstall.

Otherwise this uninstall behavior might even cause data loss (if someone links to the file via editor dialog and later removes the link).

And, yes, I know, this module's unmaintained, but I still wanted to report the problem.