Remove deprecated attributes from the MR webhook payload

The following discussion from !91606 (merged) should be addressed:

• @morefice started a discussion: (+2 comments)

  question(not-blocking): Do we have an issue somewhere to remove those deprecated fields in %16.0?

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Remove the assignee_id field from MR webhook payload: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/hook_data/merge_request_builder.rb#L64