Remove NODE_WITNESS from messages.py
Remove NODE_WITNESS from messages.py

Summary: This is not used in any test.

Test Plan:
  ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2443

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