Remove unused project/_refs partial

app/views/projects/_refs.html.haml

-= form_tag switch_project_refs_path(@project), method: :get, class: "project-refs-form" do
-  = select_tag "ref", grouped_options_refs, onchange: "this.form.submit();", class: "project-refs-select chosen"
-  = hidden_field_tag :destination, destination