Removed duplicate SIGHASH enum
Removed duplicate SIGHASH enum

Summary: Cleanup that needs to be done to continue work on T61

Test Plan: Tests pass.

Reviewers: deadalnix, schancel, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D860

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