Remove Rails Haml reference to approvers/approver_groups field on project/merge_request presenter

MR - Update reference to approvers field on merge_re... (!208035 - merged)

Description

After some research, we have found a few Rails Haml files that still reference on approvers or approver_groups through merge_request/project presenter.

There should be no harm, if we leave them there, as the current plan is to leave approvers and approver_groups methods available on merge_request/project presenter to keep API consistency. But it would be better to remove them as the current process.

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/views/notify/_merge_request_approvers.html.haml
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/views/notify/_merge_request_approvers.text.erb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/notify/new_merge_request_email.html.haml#L13