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`.

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Co-authored-by: Bastian Köcher <[email protected]>