Remove $wp_secret_key_default.

With the duplicate key checks, this is isn't necessary. It could also cause problems when you
temporarily update to an English translation of a new version before going back to the locale.

see #19599.



git-svn-id: https://core.svn.wordpress.org/trunk@20691 1a063a9b-81f0-0310-95a4-ce76da25c4cd