[lib] Delete extraneous invariant and dependency from relationship change callback

Summary:
This invariant isn't necessary, because the value isn't used in the callback. Also, the dependency on userID is still present, but indirectly through updateRelationship function.
Depends on D3732

Test Plan: Send friend request by using relationship prompt and check if the state was updated.

Reviewers: benschac, atul, def-au1t

Reviewed By: atul

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3733