documentation/poky-ref-manual: Removed the PNG files in screenshots

The directory screenshots is no longer used in this manual.  Previously,
only one file (ss-sato.png) was used and I have moved it to the
figures folder.

(From yocto-docs rev: 816aeaccbac6e89b10ffed88e47e7a9ae50778aa)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>