Deprecate the user domain tag

[tarakby]

Definition of Done

The User domain tag is not a Flow constant after the April Mainnet spork. It is also removed from the flow-go repo.

This issue is about updating the current repo to not use the User domain tag (for instance ./examples/)

cc @sideninja

[sideninja]

@tarakby is this still relevant?

[tarakby]

Yes I think so, I can still the constant is still used in SDK while it's not a FLOW constant anymore.