identity: Don't let subs be re-registered (#6667)

* Fixes and tests

* Don't set subs be re-registered.

Also allow subs to de-register themselves and collect the deposit.

Also allow individual registering and removal of subs.

* Make it build

* Update frame/identity/src/lib.rs

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Tests

* Add benchmarks

* Add some reasonable weights

* Docs

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

substrate/frame/example/src/lib.rs

@@ -723,7 +723,7 @@ mod tests {
 	};
 
 	impl_outer_origin! {
-		pub enum Origin for Test  where system = frame_system {}
+		pub enum Origin for Test where system = frame_system {}
 	}
 
 	impl_outer_dispatch! {