Avoid using Pundit.policy in _form_fields partial

We're already using the `policy` helper method elsewhere so let's use
that consistently. This also avoids us having to pass the `current_user`
in.

app/views/users/_form_fields.html.erb

@@ -47,7 +47,7 @@
       <% if @user.exempt_from_2sv? %>
         <p>
           The user has been made exempt from 2-step verification for the following reason: <%= @user.reason_for_2sv_exemption %>
-        <% if Pundit.policy(current_user, @user).exempt_from_two_step_verification? %>
+        <% if policy(@user).exempt_from_two_step_verification? %>
           <br>
           <%= link_to 'Edit reason or expiry date for 2-step verification exemption', edit_two_step_verification_exemption_path(@user) %>
         <% end %>
@@ -77,7 +77,7 @@
         <br/>
         User will be prompted to set up 2-step verification again the next time they sign in.</p>
       <% end %>
-      <% if @user.persisted? && Pundit.policy(current_user, @user).exempt_from_two_step_verification? && @user.reason_for_2sv_exemption.nil? %>
+      <% if @user.persisted? && policy(@user).exempt_from_two_step_verification? && @user.reason_for_2sv_exemption.nil? %>
           <p>
             <%= link_to 'Exempt user from 2-step verification', edit_two_step_verification_exemption_path(@user) %>
             <br/>