Removing attention request can lead to requesting attention instead
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### Summary
It was reported on [slack](https://gitlab.slack.com/archives/CETG54GQ0/p1648635892535109) (internal) that removing attention request will request an attention instead. This leads to a confusing UX.
### Steps to reproduce
From slack message:
1. I received an attention request on an MR
1. I went to the MR, submitted some comments, and then clicked on the "Remove attention request" button next to my name, thinking it would remove the attention request.
1. Later on I looked at the list of MRs that needed my attention, and that MR was still showing. In that MR's system notes, I could see that I requested my own attention.
1. What I think happened is that my initial nteractions with the MR after being assigned for review cleared the attention request, but didn't update the Reviewers UI. So, when I clicked the "Remove attention request", it simply toggled the state back to the positive, despite the UI indicating that the attention request was removed.
### What is the current *bug* behavior?
<!-- Describe what actually happens. -->
Removing attention request when it's already been removed will result to attention being requested instead.
### What is the expected *correct* behavior?
<!-- Describe what you should see instead. -->
Removing attention request when it's already been removed should not request attention instead.
### Possible fixes
Add an add/remove mutation and let the ~frontend use those instead of the toggle mutation. This way, when the UI says that the attention request was removed, it'll actually be removed.
issue