Remove deprecated assignee_id key out of Gitlab::GhookData::MergeRequestBuilder

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The following discussion from !10161 (merged) should be addressed:

• @nick.thomas started a discussion: (+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?