Drop unused desktopthumbnail.desktop
Once used to allow separate control over thumbnails for desktop files instead of being treated the same as normal plain text files, see 35aa7f41 But forgotten to be readded to installed set when the text thumbnailer was enabled again during Qt5/KF5 port, and meanwhile superseded by hardcoded code inside the text thumbnailer, see 9b27cf15
(Ideally there would be a way to not do this hardcoded in thumbnailers, as they would need to know about any possible sub-MIME-types. But that needs a principle approach, left for someone else/some other day)
Edited by Friedrich W. H. Kossebau