Twenty Thirteen: revert adding new image size for galleries., Using existing medium size instead, and thumbnail for mobile views. See #24307.

git-svn-id: http://core.svn.wordpress.org/trunk@24338 1a063a9b-81f0-0310-95a4-ce76da25c4cd
POT, generated from r24338

git-svn-id: http://core.svn.wordpress.org/trunk@24339 1a063a9b-81f0-0310-95a4-ce76da25c4cd