Hey W.,
I think creating trimmed media from the existing media is a feature thats not yet made it into FCP X.
In the meantime you could export the selections you want to keep as new self contained media files (one way would be to create a compound clip of your selection, open it into the timeline, and then export a QT movie) then bring these new clips back into your project to use in place of those original selections ... when thats done and you've double checked you've not missed anything, then you can delete the original footage.
Hope it helps
Andy