system: Stop using canonical-package.

system: Stop using canonical-package.

Usage of canonical-package outside of thunked fields breaks cross-compilation,
see: https://lists.gnu.org/archive/html/guix-devel/2019-12/msg00410.html.

* gnu/installer.scm (installer-program): Remove canonical-package.
* gnu/services/base.scm (<nscd-cache>): Ditto,
(%base-services): ditto.
* gnu/services/xorg.scm: Remove useless canonical-package import.
* gnu/system.scm (%base-packages): Remove canonical-package.
* gnu/system/install.scm (%installation-services): Ditto,
(installation-os): ditto.
* gnu/system/locale.scm (single-locale-directory): Ditto.