1) Its usually a good idea to clean the cookie cache prior to recording. Like most rules of thumb, this is not always true and you need to think about what process you are modeling and if pre-existing cookies are required to properly emulate the business transaction.
2) There is a bug in the current release of OpenSTA where the gateway (when recording) creates multiple variables to track the same cookie. Not sure if this is relevant as you never stated why you thought you had extraneous cookies.
While recording web app using OpenSTA, it's captures whatever u doing events in app and it can also captures other stuff (junk items), I want remove those junk items. I heared some java scripts is there to remove this junk but i don't no exactly what is that... I want know abt that java script.
OpenSTA doesn't record "junk", it records exactly what is passed between the browser and the system under test at the time of recording. If you want to reduce the "junk" recorded, then delete cookies and temp files using the appropriate browser command (i.e. in IE6 tools->internetOptions->DeleteCookies and (perhaps, depending on the caching behavior you want to emulate) tools->internetOptions->DeleteFiles prior to recording.
Editing a script to remove "extraneous" cookies which appear both in the data declaration section and also in gets/posts would be a tedious and error prone process. I doubt there is a script which could differentiate between "desirable" and "undesirable" cookies in a script.