This commit removes the remaining links to the Download Center on the website.
We no longer use it for installation, and we don't want users to infer that
something like that still exists.
Fixes https://github.com/scylladb/scylladb/issues/27753
Closes scylladb/scylladb#27756
(cherry picked from commit f65db4e8eb)
Closes scylladb/scylladb#27781