[qa] Delete cookie file before starting node

Summary:
If a cookie file exists in a datadir prior to node startup, it must have
been leftover from a prior unclean shutdown. As bitcoind will overwrite
it anyway, delete it before starting up to prevent the test framework
from inadvertently trying to connect using stale credentials.

Backport of core PR12902
https://github.com/bitcoin/bitcoin/pull/12902/files

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3081