Twenty Thirteen: change link URL function name to `twentythirteen_get_link_url()`, props obenland. Also remove `function_exists()` check since this function should not be pluggable, by design. Closes #23619.

git-svn-id: http://core.svn.wordpress.org/trunk@23795 1a063a9b-81f0-0310-95a4-ce76da25c4cd