Use ToHex() in rpc_decodescript.py, remove dependency to mininode
Use ToHex() in rpc_decodescript.py, remove dependency to mininode

Summary:
Avoid using the long form `bytes_to_hex_str(obj.serialize())` and remove
the global import from mininode.

Test Plan:
  ./test/functional/test_runner.py rpc_decodescript

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2536

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