Make wpmu_delete_blog() slightly less crappy. Prevents the entire network from...

Make wpmu_delete_blog() slightly less crappy. Prevents the entire network from being dropped in a particularly rare edge case. Removes some zombie code. fixes #19009.

git-svn-id: https://core.svn.wordpress.org/trunk@19038 1a063a9b-81f0-0310-95a4-ce76da25c4cd