Removed XML thumbnailing from ReviewBoard

- Removed XML thumbnailing support from ReviewBoard default ship package
  since there is now an XML Extension that integrates both the Review UI
  and Thumbnailing support for XML file attachments into ReviewBoard.
- This effectively reverses the changes in 
  http://reviews.reviewboard.org/r/3567/

Manually verified that XML file attachments now defaults back to the
TextMimetype and are handled as raw text files for thumbnailing.

(See attached screenshot)