Remove filesets.el and associated documentation

Remove filesets.el and associated documentation

This library has some nice ideas, but it was never properly
integrated into (GNU) Emacs.  We have better and more coherent
solutions these days, so its presence in tree burdens overall
maintenance with little benefit.

* doc/emacs/emacs.texi (Top)
* doc/emacs/files.texi (Files, Filesets)
* doc/emacs/maintaining.texi (Basic VC Editing): Update.
* lisp/filesets.el: Remove.