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.

gnu/system/install.scm

@@ -384,8 +384,7 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
           ;; Having /bin/sh is a good idea.  In particular it allows Tramp
           ;; connections to this system to work.
           (service special-files-service-type
-                   `(("/bin/sh" ,(file-append (canonical-package bash)
-                                              "/bin/sh"))))
+                   `(("/bin/sh" ,(file-append bash "/bin/sh"))))
 
           ;; Loopback device, needed by OpenSSH notably.
           (service static-networking-service-type
@@ -408,7 +407,7 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
                    (list bare-bones-os
                          glibc-utf8-locales
                          texinfo
-                         (canonical-package guile-2.2))))))
+                         guile-2.2)))))
 
 (define %issue
   ;; Greeting.
@@ -480,7 +479,7 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
      ;; Explicitly allow for empty passwords.
      (base-pam-services #:allow-empty-passwords? #t))
 
-    (packages (cons* (canonical-package glibc) ;for 'tzselect' & co.
+    (packages (cons* glibc ;for 'tzselect' & co.
                      parted gptfdisk ddrescue
                      fontconfig
                      font-dejavu font-gnu-unifont

gnu/system/locale.scm

@@ -86,7 +86,7 @@ or #f on failure."
      #f)))
 
 (define* (single-locale-directory locales
-                                  #:key (libc (canonical-package glibc)))
+                                  #:key (libc glibc))
   "Return a directory containing all of LOCALES for LIBC compiled.
 
 Because locale data formats are incompatible when switching from one libc to