Remove unnecessary secret_salt_warning().

If salts/keys are not defined in wp-config.php then they will be generated
automatically and stored in the database. [19771] also deals with values
that are duplicated or set to default.

Fixes #14803.


git-svn-id: https://core.svn.wordpress.org/trunk@24813 1a063a9b-81f0-0310-95a4-ce76da25c4cd