Create LegacyScriptPubKeyMan when not a descriptor wallet
Summary:
Backport of Core PR16528 part [5/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/6b13cd3fa854dfaeb9e269bff3d67cacc0e5b5dc
Depends on D8401
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8403