Update blog last_updated time only on publish_post. Both private_to_published...

Update blog last_updated time only on publish_post. Both private_to_published and publish_phone are overly broad and otherwise redundant. props mdawaffe, fixes #18624.

git-svn-id: https://core.svn.wordpress.org/trunk@18657 1a063a9b-81f0-0310-95a4-ce76da25c4cd