Fixes <http://bugs.gnu.org/22607>.
Reported by myglc2 <myglc2@gmail.com>.
Remove possibilities to install/delete packages to/from a system profile
both for "Package List" and "Package Info" buffers.
* emacs/guix-profiles.el (guix-system-profile-regexp): New variable.
(guix-system-profile?): New procedure.
* emacs/guix-ui-package.el (guix-package-info-insert-output): Do not
display "Install"/"Delete" button for a system profile.
(guix-package-assert-non-system-profile): New procedure.
(guix-package-execute-actions): Use it.