Remove SingleSHA1Hash

[peterebden]

Bit of coding to pass the time on the plane. Removes the SingleSHA1 fflag and associated code. This simplifies these code paths a bit for the next change (which is the real thing I wanted, but split this out to simplify it a bit).

Should be a v17 change since we never made this the default in v16.

[stale[bot]]

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[peterebden]

Bad bot, we're waiting to make breaking changes.

[stale[bot]]

This issue has been automatically marked as stale because it has not had any recent activity in the past 90 days. It will be closed if no further activity occurs. If you require additional support, please reply to this message. Thank you for your contributions.