doc: remove unused figures from board directory

Board documentation was moved to wiki.zephyrproject.org and documents with
links to the corresponding wiki article left in their place.  This fix
cleans up the figures (.png files) that were left behind and are no longer
needed here.

Change-Id: I9056046fd7c9307de750360e20d8f970ee7ae3b9
Jira: ZEP-564
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>