Make push only a standard flag.
Make push only a standard flag.

Summary: As it turns out IsStandardTx already checks for it, so there is no semantic change.

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

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1622

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