Rootstock: revert HOCON env-var sub - hardcode rpc-de-31 domain

${?DOMAIN} substitution caused rskj to crash with 'Can't read config'.
Hardcoding the host-specific domain for now (rootstock only runs on
de-31). For multi-host deployment later, would need a per-host
templated node.conf or a proxy that rewrites Host header.

rootstock/mainnet/node.conf

@@ -11,9 +11,7 @@ rpc {
             http = {
                 enabled = true
                 bind_address = "0.0.0.0"
-                # Whitelist: include localhost variants AND the public domain.
-                # ${?DOMAIN} is HOCON env-var sub: skipped if env var is unset.
-                hosts = ["localhost", "127.0.0.1", "::1", ${?DOMAIN}]
+                hosts = ["localhost", "127.0.0.1", "::1", "rpc-de-31.stakesquid.eu"]
                 port = 8545
             }
             ws = {