[backport#17304 6/18] Remove SetWalletFlag from WalletStorage

Summary:
SetWalletFlag is unused.

Does not change any behavior

https://github.com/bitcoin/bitcoin/pull/17304/commits/0391aba52dcc33613295fd6099c804ac7134b063

Depends on D7152

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7153