Remove direct invocations of ctrl.Finish() when using gomock

The following discussion from !1694 (merged) should be addressed:

• @jaime started a discussion: (+1 comment)

  non-blocking: we could use t.Cleanup in the tests context here. Something to consider along with some future refactoring !1694 (comment 2010582706)

  	t.Cleanup(ctrl.Finish)

Since https://github.com/golang/mock/pull/422, we no longer need to call Finish explicitly.