Remove unnecessary forward declaration
Remove unnecessary forward declaration

Summary: This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/17431 | PR17431]]

Test Plan: `ninja`

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8261

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