Update login-logo.png to use proper color. Copy it over wordpress-logo.png....

Update login-logo.png to use proper color. Copy it over wordpress-logo.png. Make that one canonical, and stop using login-logo.png. Leave it for plugins. props iammattthomas. see #19955.



git-svn-id: https://core.svn.wordpress.org/trunk@19937 1a063a9b-81f0-0310-95a4-ce76da25c4cd