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

@@ -44,7 +44,7 @@
 //!
 //! ```
 //! use frame_support::{decl_module, dispatch};
-//! use frame_system::{self as system, ensure_signed};
+//! use frame_system::ensure_signed;
 //! use pallet_im_online::{self as im_online};
 //!
 //! pub trait Trait: im_online::Trait {}
@@ -97,7 +97,7 @@ use frame_support::{
 	traits::Get,
 	weights::Weight,
 };
-use frame_system::{self as system, ensure_none};
+use frame_system::ensure_none;
 use frame_system::offchain::{
 	SendTransactionTypes,
 	SubmitTransaction,