Recursive include of the XWiki.ClassSheet on XWiki.Mail

The old mail sender plugin is still being initialize and, as part of its initialization, it also initializes the XWiki.Mail class which is already handled by MailMandatoryDocumentInitializer. While doing so, it also sets a manual include in the content of the class document, which results in the recursive include of XWiki.ClassSheet.

The fix is to remove the mail sender plugin's class initialization, however, we should also investigate why the sheet module fails to handle this case correctly, since this might be a backwards compatibility issue.