Drop OfficialClientManager and references to it

OfficialClientManager is unused now that scenario tests use
clients.Manager instead. Dropping the class, the hacking
rule that forbade its use in API tests and related unit tests.

Change-Id: Ib359b9ab0a903436d9d9c265e8c4d82bdd80a496
Merge "Drop OfficialClientManager and references to it"