[backport#15492] [rpc] remove deprecated generate method

Summary:
[rpc] remove deprecated generate method
https://github.com/bitcoin/bitcoin/pull/15492/commits/8bb3e4c487500a580e3e18791b1f4e7dcdd35442

[wallet] remove unused GetScriptForMining
https://github.com/bitcoin/bitcoin/pull/15492/commits/07cae5287c4409e9e597b891899403220905881d

Update: considering this is a testing only RPC and it is blocking a bunch of current stuff (and part of a PR going back to D6269), discussion has been mostly in favor of shipping this a bit over a month ahead of the normal deprecation schedule.

Backport of Core PR15492

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6710