decl_module! macro: use 'frame_system' instead of system as default ident (#6500)

* Use frame_system as default ident.

* Remove unused 'frame_system' to 'system' renaming.

* Fix construct_runtime_ui tests.

* Rename system to frame_system in sudo/utility pallet test.

* Bump runtime impl_version.

* Update formatting.

Author: shaunxw

src/lib.rs

@@ -47,7 +47,7 @@ use frame_support::{
 	decl_module, decl_event, decl_storage, ensure, decl_error,
 	traits::{Currency, EnsureOrigin, ReservableCurrency, OnUnbalanced, Get},
 };
-use frame_system::{self as system, ensure_signed};
+use frame_system::ensure_signed;
 
 type BalanceOf<T> = <<T as Trait>::Currency as Currency<<T as frame_system::Trait>::AccountId>>::Balance;
 type NegativeImbalanceOf<T> = <<T as Trait>::Currency as Currency<<T as frame_system::Trait>::AccountId>>::NegativeImbalance;