Remove traces of "GuixSD".

Remove traces of "GuixSD".

* gnu/bootloader/extlinux.scm (extlinux-configuration-file): Remove
mentions of "GuixSD".
* gnu/bootloader/grub.scm (install-grub-efi): Likewise.
* gnu/build/vm.scm (make-iso9660-image): Change default #:volume-id to
"Guix_image".
(initialize-hard-disk): Search for the "Guix_image" label.
* gnu/ci.scm (system-test-jobs, tarball-jobs): Remove "GuixSD".
* gnu/installer/newt/welcome.scm (run-welcome-page): Likewise.
* gnu/packages/audio.scm (supercollider)[description]: Likewise.
* gnu/packages/curl.scm (curl): Likewise.
* gnu/packages/emacs.scm (emacs): Likewise.
* gnu/packages/gnome.scm (network-manager): Likewise.
* gnu/packages/julia.scm (julia): Likewise.
* gnu/packages/linux.scm (alsa-plugins): Likewise.
(powertop, wireless-regdb): Likewise.
* gnu/packages/package-management.scm (guix): Likewise.
* gnu/packages/polkit.scm (polkit): Likewise.
* gnu/packages/tex.scm (texlive-bin): Likewise.
* gnu/services/base.scm (file-systems->fstab): Likewise.
* gnu/services/cups.scm (%cups-activation): Likewise.
* gnu/services/mail.scm (%dovecot-activation): Likewise.
* gnu/services/messaging.scm (prosody-configuration)[log]: Likewise.
* gnu/system/examples/vm-image.tmpl (vm-image-motd): Likewise.
* gnu/system/install.scm (installation-os)[file-systems]: Change root
file system label to "Guix_image".
* gnu/system/mapped-devices.scm (check-device-initrd-modules):
Remove "GuixSD".
* gnu/system/vm.scm (system-docker-image): Likewise.
(system-disk-image)[root-label]: Change to "Guix_image".
* gnu/tests/install.scm (run-install): Remove "GuixSD".
* guix/modules.scm (guix-module-name?): Likewise.
* nix/libstore/optimise-store.cc: Likewise.