Remove support for image thumbnails

[ace-dent]

For efficient, web-based images - we should never write a thumbnail image. Strip out support from code for thumbnails.

[bdaehlie]

Lets just make sure it's off by default rather than ripping the code out for now.

[kornelski]

Ouch. I've realized libjpeg-turbo commits with issue numbers in them act on wrong issues in this repo! Sorry!