Remove modal-ok and modal-cancel slots from modal

As noted in this discussion, it seems that the modal-ok and modal-cancel slots were broken in !767 (merged), and no longer work.

Since they no longer work, they should simply be removed.

Affected gitlab-org/gitlab components

• https://gitlab.com/gitlab-org/gitlab/blob/93d77316b5d03f06fd6e38f7dceb577b2df9977b/app/assets/javascripts/monitoring/components/duplicate_dashboard_modal.vue#L90
• https://gitlab.com/gitlab-org/gitlab/blob/93d77316b5d03f06fd6e38f7dceb577b2df9977b/app/assets/javascripts/monitoring/components/alert_widget_form.vue#L315
• https://gitlab.com/gitlab-org/gitlab/blob/93d77316b5d03f06fd6e38f7dceb577b2df9977b/app/assets/javascripts/packages/list/components/packages_list.vue#L120
• https://gitlab.com/gitlab-org/gitlab/blob/f9ac955065d14c4d5f874df5390f78713753a472/ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/approvals_auth.vue#L88

Implementation plan

• Replace usage of modal-ok and modal-cancel in gitlab-org/gitlab with either modal-footer or (primary|secondary)Action props (see above for list of affected components). These have likely been broken for a while, so perhaps instead the usage of these slots can simply be removed? gitlab!87415 (merged)
• Remove modal-ok and modal-cancel slots from modal component !2827 (merged)