Remove "guile-zlib" extension when unused.

This is a follow-up of 755f365b.

As (zlib) is autoloaded in (gnu build linux-modules), "guile-zlib" is needed
as an extension only when it is effectively used.

* gnu/installer.scm (installer-program): Remove "guile-zlib" from the extensions.
* gnu/machine/ssh.scm (machine-check-initrd-modules): Ditto.
* gnu/services.scm (activation-script): Ditto.
* gnu/services/base.scm (default-serial-port): Ditto,
(agetty-shepherd-service): ditto,
(udev-service-type): ditto.
* gnu/system/image.scm (gcrypt-sqlite3&co): Ditto.
* gnu/system/shadow.scm (account-shepherd-service): Ditto.