Make init_logging more easily usable (#6620)

Instead of requiring the `LogRotationOpt`, it now requires an
`Option<LogRotationOpt>`. This makes it much more easy to use the
interface when someone isn't interested on the `LogRotationOpt`'s

substrate/bin/node/bench/src/main.rs

@@ -63,9 +63,6 @@ struct Opt {
 	#[structopt(long)]
 	transactions: Option<usize>,
 
-	#[structopt(flatten)]
-	log_rotation_opt: sc_cli::LogRotationOpt,
-
 	/// Mode
 	///
 	/// "regular" for regular benchmark
@@ -80,7 +77,7 @@ fn main() {
 	let opt = Opt::from_args();
 
 	if !opt.json {
-		sc_cli::init_logger("", &opt.log_rotation_opt).expect("init_logger should not fail.");
+		sc_cli::init_logger("", None).expect("init_logger should not fail.");
 	}
 
 	let mut import_benchmarks = Vec::new();