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