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/file/setup.go

@@ -113,8 +113,8 @@ func fileParse(c *caddy.Controller) (Zones, error) {
 				reload = d
 
 			case "upstream":
-				// ignore args, will be error later.
-				c.RemainingArgs() // clear buffer
+				// remove soon
+				c.RemainingArgs()
 
 			default:
 				return Zones{}, c.Errf("unknown property '%s'", c.Val())