make polkadot-runtime optional feature (#3820)

* make polkadot-runtime optional feature

* sprinkle some cfg statements

* ok

* ok, ok

* add CI check

* set -e

* chmod +x

* fixes

* fmt

* nicer compile errors

* Update outdated comments

polkadot/scripts/gitlab/check_no_default_features.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+set -e
+
+pushd node/service && cargo check --no-default-features && popd
+pushd cli && cargo check --no-default-features --features "service" && popd