fbdev has been deprecated and unmaintained for a while. With Linux 5.14
including SimpleDRM driver, we can drop it. (at the time of writing this
commit message, the latest Linux version is 5.16).
This improves file organization in kwin by putting backends in a single
directory.
It also makes easier to discover kwin's low level components for new
contributors because the plugins directory may come as the last place to
look for. When one hears "plugin", the first thing that comes to mind is
regular plugins, not low level backends.
