remove indentation level in ProcessGetBlockData

Summary:
This concludes backport of core#21235
https://github.com/bitcoin/bitcoin/pull/21235/commits/fa8177324392c923c6ce39056cfd870af55ab673

Depends on D13310

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13311