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

@@ -64,7 +64,7 @@
 //! ```
 //! use frame_support::{decl_module, dispatch};
 //! # use pallet_timestamp as timestamp;
-//! use frame_system::{self as system, ensure_signed};
+//! use frame_system::ensure_signed;
 //!
 //! pub trait Trait: timestamp::Trait {}
 //!