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Fixes failing job back off test

**What this PR does / why we need it**:

 The test `TestJobBackoff` is always failing on my local environment.

After debugging, I find that it is caused by `fakeRateLimitQueue` in which the `Forget` call should have delete the key entry from the queue. But actually it is doing nothing. 

IDK why upstream test is running successfully all the time. Am I missing sth?


**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```