net: Simplify ProcessGetBlockData execution by removing send flag.

Summary:
Setting the send flag to false can be replaced by simply returning.

This is a partial backport of core#21235
https://github.com/bitcoin/bitcoin/pull/21235/commits/fae77b9e6dc9e59b355d56df49c4d9685b6f40a4

Depends on D13309

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13310