Remove PID file at the very end

Summary:

While reproducing the bug from #18517, I've noticed that the bitcoind.pid file has already been removed when the bitcoind hangs.
This PR makes Shutdown() keep the bitcoind.pid file available until the end.

This is a backport of Core PR18526

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8910