Revert "gui: Do not exit and re-enter main event loop during shutdown"

Summary:
This reverts commit rABC1219aeddf633c88c43fb226f9401ef1e1bcc4ce7.

This is causing a crash of Bitcoin-Qt upon shutdown on MacOS.
This is also the cause of another bug: https://github.com/bitcoin-core/gui/pull/509

Test Plan: Check the MacOS crash is fixed.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18166