Revert mining changes in #5957
This reverts commit e2edf95cd3f43331843676e49a82830128a95050 6b04508e37c5dd18cec1cd61cc4356bd208aa991 0df67f1f7ab4adfe9f0b3ba6276e737b37826464,
except the changes to the RPC tests.
A generate RPC call is introduced based on the old code.