Remove response check in Delete* call tests

[artek-koltun]

In many Delete* calls, a response is checked as

if reflect.TypeOf(response) != reflect.TypeOf(tt.out) {
    t.Error("response: expected", reflect.TypeOf(tt.out), "received", reflect.TypeOf(response))
}

However, this check does not make much sense. response is returned from a Delete* client function call, which has a concrete type *emptypb.Empty. Any other type cannot be returned from there, since Go is a statically typed language.
Then response is compared against an expectation referred also as *emptypb.Empty. Thus, we just compare that response's type *emptypb.Empty is equal to out's type *emptypb.Empty. The result is always true.

Also, emptypb.Empty represents an empty message in gRPC. It does not have any exported fields, and we do not have anything to check there.