net: Remove unused interrupt from SendMessages

Summary:
And also from ProcessGetBlockData(...).

Backport of core#13615.

Ref T1696.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10879