fix assert crash when specified change output spend size is unknown

Summary:
Remove stale comment in CalculateMaximumSignedInputSize

CreateTransaction: Assume minimum p2sh-p2wpkh spend size for unknown change

This is a backport of Core PR14380

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8500