Revert approval voting (#5438)

* Revert approval-voting subsystem

* Approval voting revert encapsulated within 'ops' module

* use 'get_stored_blocks' to get lower block height

* Fix error message

* Optionally shrink/delete stored blocks range

* range end number is last block number plus 1

* Apply code review suggestions

* Use tristate enum for block range in backend overlay

* Add clarification comment

* Add comments to private struct

polkadot/node/core/approval-voting/src/tests.rs

@@ -306,6 +306,9 @@ impl Backend for TestStoreInner {
 				BackendWriteOp::WriteStoredBlockRange(stored_block_range) => {
 					self.stored_block_range = Some(stored_block_range);
 				},
+				BackendWriteOp::DeleteStoredBlockRange => {
+					self.stored_block_range = None;
+				},
 				BackendWriteOp::WriteBlocksAtHeight(h, blocks) => {
 					self.blocks_at_height.insert(h, blocks);
 				},