plugins: set upstream unconditionally (#2956)

`upstream` is not needed as a setting; just set if unconditionally and
remove all documentation and tests for it.

At some point we want remove the hanlding for `upstream` as well and
error out on seeing it.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/federation/README.md

@@ -17,15 +17,11 @@ Enabling *federation* without also having *kubernetes* is a noop.
 ~~~
 federation [ZONES...] {
     NAME DOMAIN
-    upstream
 }
 ~~~
 
 * Each **NAME** and **DOMAIN** defines federation membership. One entry for each. A duplicate
   **NAME** will silently overwrite any previous value.
-* `upstream` resolve the `CNAME` target produced by this plugin.  CoreDNS
-  will resolve External Services against itself and needs the *forward* plugin to be active to do
-  so.
 
 ## Examples
 
@@ -37,7 +33,6 @@ Here we handle all service requests in the `prod` and `stage` federations.
     federation cluster.local {
         prod prod.feddomain.com
         staging staging.feddomain.com
-        upstream
     }
     forward . 192.168.1.12
 }