guix gc: '--delete-generations' now deletes old Home generations.

guix gc: '--delete-generations' now deletes old Home generations.

This reverts commit 24c0518dd404cbb3c434fb6704f4f551bbc78693,
thereby reinstating ba22560627f848f40891a56355ff26b6de1380bc, with an
additional fix in (guix self).

Fixes <https://issues.guix.gnu.org/56722>.
Reported by "(" <paren@disroot.org>.

* guix/scripts/gc.scm (guix-gc)[delete-generations]: Add call to
'home-generation-base'.
* guix/self.scm (compiled-guix)[*core-cli-modules*]: Remove (guix
scripts gc).
* doc/guix.texi (Invoking guix gc): Document the change.