[backport#15492 1/2] [rpc] simplify generate RPC

Summary:
Removes dead code from after the generate method was removed

https://github.com/bitcoin/bitcoin/pull/15748/commits/9819ad6d07097ca4df92231e18de0b57313e3b78

Depends on D6710

Partial backport of Core PR15748

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6712