gnu: emacs: Simplify the EMACSLOADPATH search path specification.

gnu: emacs: Simplify the EMACSLOADPATH search path specification.

The EMACSLOADPATH can be greatly simplified by relying on a subdirs.el file
that causes Emacs to search recursively a directory found in EMACSLOADPATH.

* gnu/packages/emacs.scm (emacs)[native-search-paths]: Remove the match-all
file pattern regexp.  Remove the versioned site-lisp directory from searched
files, as it appears unused by Emacs.

Reported-by: Leo Prikler <leo.prikler@student.tugraz.at>
Signed-off-by: Clément Lassieur <clement@lassieur.org>