cling binary release is obsolete

[ferdymercury]

Here, https://root.cern/cling/cling_build_instructions/, it is said:

You can find our nightly binary snapshots here

But the latest nightly is from 2020.

So this should be either removed or the CI release procedure fixed to include latest nightlies.

Related: https://github.com/root-project/web/issues/412