desktop.el: Janitorial cleanup

desktop.el: Janitorial cleanup

* lisp/desktop.el: Remove redundant `:group` arguments.
Prefer #' to quote function names.
(desktop-var-serdes-funs): Improve docstring markup.
(desktop-clear, desktop-create-buffer): Use lexical-binding.
(desktop--v2s): Avoid (ab)use of ,@ which doesn't splice a list.
(desktop-outvar): Strength-reduce `eval` => `symbol-value`.
(desktop-create-buffer): Don't assume point-min==1.
(desktop-buffer): Belatedly mark as obsolete.
(desktop-read): Use `locate-file`.

(cherry picked from commit 2bf3790f374e0a162e05a470940f57b43a244ad7)