website: Get rid of tables for snapshots in where/

Reduce the number of seemingly identical tables on the downloads
page by half by only providing them for release images. The utility
of those tables for snapshots was already questionable, with eg
differently individual links leading to the same target directory.

Reviewed By: dbaio (docs), gjb (releng)
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D34134