[backport#16415] Get rid of PendingWalletTx class.

Summary:
No reason for this class to exist if it doesn't have any code to run in the
destructor. e10e1e8db043e9b7c113e07faf408f337c1b732d from
https://github.com/bitcoin/bitcoin/pull/16208 recently removed code destructor
code that would return an unused key if the transaction wasn't committed.

https://github.com/bitcoin/bitcoin/pull/16415/commits/4d94916f0dda535cb69b538ee4e3fffb5b033c87

Backport of Core PR16415

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6804