Remove RuntimeApi dependency on system parachain runtime code (#2455)

The last issue blocking the removal of the Polkadot and Kusama system
parachains from the repo in #1737 is the dependency on the runtime code
through the RuntimeApi in `polkadot-parachain`.

This PR introduces two fake runtimes to satisfy the build requirements
and changes the `new_partial` function to make it not be generic over
the runtimes.
The reason for the second runtime is the different Aura keys used in
Polkadot Asset Hub, as the impl for AuraApi depends on this type.
If this changes the `RuntimeApi` generic could be removed completely
from all functions in `services.rs` and and generic type parameters in
`services.rs` and specified as a concrete type to TFullClient`.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/polkadot-parachain/src/fake_runtime_api/mod.rs

@@ -0,0 +1,21 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Cumulus is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Cumulus is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+//! In an ideal world this would be one runtime which would simplify the code massively.
+//! This is not an ideal world - Polkadot Asset Hub has a different key type.
+
+pub mod asset_hub_polkadot_aura;
+pub mod aura;