Remove deprecated assignee_id key out of Gitlab::GhookData::MergeRequestBuilder
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The following discussion from !10161 should be addressed:
- [ ] @nick.thomas started a [discussion](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161#note_157011671): (+1 comment)
> I guess this is a backward-incompatible change? Is it work keeping `assignee` in place for merge requests (and adding a follow-up issue to remove in %12.0), as well as adding the new `assignees` member?
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