Remove redundant stopExecutor() signal

Summary:
PR description:

The QThread::finished signal do this work.

This concludes backport of Core PR14250

Commit 24313fb

Depends on D7844

Test Plan:
ninja && ninja check

Run src/qt/bitcoin-qt -regtest, open the console and run some RPC commands,
make sure the shutdown works without issues.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7845