Remove redundant stopThread() signal

Summary:
PR description:

The QThread::finished signal do this work.

Backport of Core PR14250, part 1 of 2
Commit 1c0e0a5

Test Plan:
ninja && ninja check

Then run src/qt/bitcoin-qt and make sure there are no errors on shutdown.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7844