In order to start wifi syncing with a new computer, you must first attach your iPad, over your charging cable, to the computer with iTunes running. Then select 'Sync this iPad over Wifi' in the Options in the iPad Summary screen in iTunes.
To get rid of a sync 'host', you need to connect your iPad to iTunes on the host, then deselect 'Sync this ipad over wifi' in the Options of the iPad Summary of iTunes.
Having one 'host' does not impact the ability to sync over wifi to another one. The iPad will sync with whatever machine is online and available over wifi. If the older host no longer exists, this won't impact syncing your iPad to the new computer at all, and other than connecting it to that computer, there is no way to remove that item on your Ipad.