Don't touch web.config when flushing rewrite rules if it is a multisite...

Don't touch web.config when flushing rewrite rules if it is a multisite install. Fixes #16076 props nacin.

git-svn-id: https://core.svn.wordpress.org/trunk@17328 1a063a9b-81f0-0310-95a4-ce76da25c4cd