The ability to uninstall Project Server or Remove Project Server from a SharePoint Farm is not supported, regardless if it’s 2007 or 2010.
2007 Removing either Office SharePoint Server 2007 or Office Project Server 2007 from a joint installation is not supported. Installing Office SharePoint Server 2007 with Office Project Server 2007 extends the farm schema, and it cannot be undone by uninstalling one of the applications. http://technet.microsoft.com/en-us/library/cc197677(office.12).aspx
2010 Once you install Project Server 2010, it cannot be uninstalled from the farm. If you want to remove Project Server 2010 functionality, you can turn off the Project Application Service and delete the Project Server 2010 Service Application. http://technet.microsoft.com/en-us/library/ee662109.aspx