[Chronik] Remove unneeded .as_ref() call

Summary:
This call is unneeded and magically breaks the type inference when plugins are enabled.

Might be a compiler bug; but since this is a good change regardless there's little point in investigating deeper.

Test Plan: ninja

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16530