remove unused UpdateCoins declaration
remove unused UpdateCoins declaration

Summary: There are two other versions of `UpdateCoins` a few lines below, one of which has the same parameters but in a different order.

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11188

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