Remove redundant code in MutateTxSign(CMutableTransaction&, const std::string&)
Remove redundant code in MutateTxSign(CMutableTransaction&, const std::string&)

Summary: This is a backport of Core PR10694

Test Plan:
  make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3337

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