[backport#17203] wallet: Remove unused GetLabelName

Summary:
wallet: Remove unused GetLabelName (Sebastian Falbesoner)

Pull request description:

While taking a look at #17198 I noticed that the method `CWallet::GetLabelName(...)` is not used anymore, since the `account` API was removed (c9c32e6b844fc79467b7e24c6c916142a0d08484).

https://github.com/bitcoin/bitcoin/pull/17203/commits/7ca68e1461376afc22a26268c80a7281522ed9eb

Backport of Core PR17203

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7093