release/Makefile.vm: Garbage collect unused bits

The cw*-package targets were introduced in February 2015 as part of
adding support for building GCE images; but GCE support was reworked
in June 2015 and the cw*-package targets were in fact never used.
Remove them.

The *_DISK variables were introduced in February 2015 as part of
adding the cloudware-install target; this was removed in May 2016 as
the cloudware images are published via the respective cloud systems
and not published as disk images via the FreeBSD FTP site. As such,
the *_DISK variables are not unused; remove them.

Approved by: re (gjb)
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41789

(cherry picked from commit fcbeeb5ff456b2f21d19d4fc6221774b512e30b7)
(cherry picked from commit b9013d75c8531e1eaa993321a22eb7714af10f68)