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

@@ -28,7 +28,7 @@ use frame_support::{
 	decl_module, decl_storage, decl_event, decl_error,
 	traits::{ChangeMembers, InitializeMembers, EnsureOrigin, Contains},
 };
-use frame_system::{self as system, ensure_signed};
+use frame_system::ensure_signed;
 
 pub trait Trait<I=DefaultInstance>: frame_system::Trait {
 	/// The overarching event type.