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 <git-user-email.h0ly5@simplelogin.com>
Co-authored-by: Bastian Köcher <git@kchr.de>