rpc: Remove unnecessary casting of block height

Summary:
One of the two occurences in Bitcoin Core was already fixed in Bitcoin ABC.

This is a backport of core#21391 [13/14]
https://github.com/bitcoin/bitcoin/pull/21391/commits/f99913969f92b8b9cef1b83f5ee8e6a9267b4af0

Depends on D11587

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11588