remove files_imageviewer

The imageviewer is now officially deprecated in favor of the slideshow implementation that comes with the gallery app. It also works in the filelis and public file shares. This PR removes the deprecated app.

Extracting the slideshow as a standalone image viewer app is possible but should be done in a separate PR. contributions welcome.

@karlitschek @jancborchardt @DeepDiver1975 @kabum @icewind1991 kill it with fire