As far as I know, you shouldn't need to deploy the System.Web.Extension dll with a SharePoint 2010 wsp package - SharePoint 2010 already offer Ajax support ootb, so the dll you are referencing should be present on the server.
Furthermore, IF the need to deploy aforementioned dll arise, you should consider to install it manually. If you use a wsp package, you may have problem in the future if you need to retract/unistall the solution (when retracted, a solution will remove all of its deployed dll, even if some of them are used by other solutions), so perhaps it's safer if you find another way to deploy the dll.
That said you should:
- confirm that the System.Web.Extensions.dll is available in gac on your test environment
- remove the dll from the deploy list in your visual studio project (leave a comment if you need guidance)