Proposal - remove `CustomGroups` section from default contribution backoffice receipt

I'm proposing we remove the following chunk of code from the back-office contribution receipt where it has not been customised

     {if !empty($customGroup)}
      {foreach from=$customGroup item=value key=customName}
       <tr>
        <th {$headerStyle}>
         {$customName}
        </th>
       </tr>
       {foreach from=$value item=v key=n}
        <tr>
         <td {$labelStyle}>
          {$n}
         </td>
         <td {$valueStyle}>
          {$v}
         </td>
        </tr>
       {/foreach}
      {/foreach}
     {/if}

This code adds details about all the custom fields associated with the contribution. For many sites this is not an issue as there are none :-)

However, for those that have contribution custom fields there are generally a mix of fields that are public facing and private facing (I've seen organisations accidentally send out commission information in the past).

It is now possible to use contribution tokens in all contribution workflow templates so the idea would be that

• if the offline template is uncustomised and the site has contribution custom fields they get a pre-upgrade message
• the message would say that if they upgrade the above section will be removed & emails would not have custom field information going forwards and point them to version-specific-upgrade-docs
• the upgrade docs would offer two options (not recommended) - put back the above block, which may not be supported forever or (recommended) construct the fields they want with tokens on the 'pdf letter for contributions screen' and copy it back