replace cli with engine method, simplify

parity/cli.rs

@@ -163,9 +163,6 @@ Sealing/Mining Options:
   --reseal-min-period MS   Specify the minimum time between reseals from
                            incoming transactions. MS is time measured in
                            milliseconds [default: 2000].
-  --internal-sealing       Use an internal sealing mechanism,
-                           suitable for PoA or PoS.
-
   --work-queue-size ITEMS  Specify the number of historical work packages
                            which are kept cached lest a solution is found for
                            them later. High values take more memory but result
@@ -369,7 +366,6 @@ pub struct Args {
 	pub flag_force_sealing: bool,
 	pub flag_reseal_on_txs: String,
 	pub flag_reseal_min_period: u64,
-	pub flag_internal_sealing: bool,
 	pub flag_work_queue_size: usize,
 	pub flag_remove_solved: bool,
 	pub flag_tx_gas_limit: Option<String>,

parity/configuration.rs

@@ -326,7 +326,6 @@ impl Configuration {
 		let options = MinerOptions {
 			new_work_notify: self.work_notify(),
 			force_sealing: self.args.flag_force_sealing,
-			internal_sealing: self.args.flag_internal_sealing,
 			reseal_on_external_tx: reseal.external,
 			reseal_on_own_tx: reseal.own,
 			tx_gas_limit: match self.args.flag_tx_gas_limit {