operations: Fix flaky UserRebaseConfirmable tests

Our tests exercising the UserRebaseConfirmable RPC are failing in CI for the Go implementation. The failure is a simple race: we're awaiting that the RPC wraps up and returns an EOF error, but it's not fast enough to do so in one second. This most frequently happens in the code coverage job, and most notably it only happens for the Go port. This is a bit puzzling considering that the Go port is supposed to be faster given that it's not creating a worktree like the Ruby implementation does. But the fact that it is the code coverage job which is hitting the issue hints at the fact that we hit in Go only because there's more instrumentation to be able to determine which code paths have been hit, which naturally doesn't exist in the Ruby port.

In any case, it doesn't really help to assert that the RPC finishes in a fixed amount of time here. So let's fix flakiness by just dropping the timeouts.