emacs: Remove 'generation' search type.

Use 'installed' search type instead.

* emacs/guix-main.scm (%patterns-makers): Remove 'generation'.
  (package/output-sexps): Adjust accordingly.
* emacs/guix-ui-generation.el (guix-generation-info-insert-number):
  Replace 'generation' search with 'installed'.
  (guix-generation-list-show-packages): Likewise.
* emacs/guix-messages.el (guix-messages): Remove 'generation'.