[backport#17304 7/18] Refactor: Remove UnsetWalletFlag call from LegacyScriptPubKeyMan::SetHDSeed

Summary:
This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/78e7cbc7ba5938ab2ae6347141ca793a8fc09853

Depends on D7153

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7154