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/kubernetes/setup_test.go

@@ -381,21 +381,6 @@ func TestKubernetesParse(t *testing.T) {
 			podModeDisabled,
 			fall.F{Zones: []string{"ip6.arpa.", "inaddr.arpa.", "foo.com."}},
 		},
-		// Valid upstream
-		{
-			`kubernetes coredns.local {
-	upstream
-}`,
-			false,
-			"",
-			1,
-			0,
-			defaultResyncPeriod,
-			"",
-			"",
-			podModeDisabled,
-			fall.Zero,
-		},
 		// More than one Kubernetes not allowed
 		{
 			`kubernetes coredns.local