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 <[email protected]>

* Tests

* Add benchmarks

* Add some reasonable weights

* Docs

Co-authored-by: joe petrowski <[email protected]>