removed scheme from admin_url to allow it to automatically choose

removed scheme from admin_url to allow it to automatically choose the correct scheme based on FORCE_SSL_ADMIN and is_ssl()

When FORCE_SSL_ADMIN is enabled, but the current page is not SSL, the "(is_ssl() ? 'https' : 'http')" logic causes an "http" URL to be generated when an "https" is supposed to be used.

http://codex.wordpress.org/Function_Reference/admin_url