Removes the /blog slug from permalinks when you use multisite in subdirectory mode.
Install and activate on site ID 1. After flushing permalinks, go to Settings -> Remove /blog Slug and the options will be updated so permalinks will function without that slug forcefully prepended.
You should only use this when you know there will be no manual url collisions. ie, network with domain mapping or if you're using a multi network plugin with no sites hanging off the main network/url.