[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
---
Backport of Core PR15492