remove out-of-date comment on pay-to-witness support
remove out-of-date comment on pay-to-witness support

Summary: This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/15833 | PR15833]]

Test Plan: Proof-reading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8365

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