Remove unused validators.Manager mock (#2944)

Author: StephenButtolph

scripts/mocks.mockgen.source.txt

@@ -1,7 +1,6 @@
 snow/engine/common/sender.go=StateSummarySender,AcceptedStateSummarySender,FrontierSender,AcceptedSender,FetchSender,AppSender,QuerySender,CrossChainAppSender,NetworkAppSender,Gossiper=snow/engine/common/mock_sender.go
 snow/networking/router/router.go=InternalHandler=snow/networking/router/mock_router.go
 snow/networking/sender/external_sender.go==snow/networking/sender/mock_external_sender.go
-snow/validators/manager.go=SetCallbackListener=snow/validators/mock_manager.go
 vms/avm/block/executor/manager.go==vms/avm/block/executor/mock_manager.go
 vms/avm/txs/tx.go==vms/avm/txs/mock_unsigned_tx.go
 vms/platformvm/block/executor/manager.go==vms/platformvm/block/executor/mock_manager.go