Parachain Template: Prune `AuxStore` bound and corresponding crate dependency (#2303)

This small PR removes an unnecessary trait bound to the `AuxStore` trait
from the Parachain template's `rpc.rs` file.

With that bound removed, the entire dependency on `sc-client-api` can
also be removed.

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