Remove the "Describe this ___" string on captions in the attachment details...

Remove the "Describe this ___" string on captions in the attachment details view. WP <3 patches with net red. props helenyhou. fixes #22777

git-svn-id: https://core.svn.wordpress.org/trunk@23097 1a063a9b-81f0-0310-95a4-ce76da25c4cd