How can we remove unused clips from Final Cut X?

When you select a clip and press Delete you are "Rejecting" that clip, not actually deleting it. To see Rejected clips, in the browser filter menu, choose Rejected. To see everything that hasn't been Rejected, choose Hide Rejected.

To show used sections of clips, choose View->Browser->Used Media Ranges

Here's a good explanation: https://larryjordan.com/articles/find-unused-media-in-final-cut-pro/

If you really want to delete a clip, right-click on it and choose Move to Trash. Be careful about what you're doing so that you don't unintentionally delete the clip from your Mac if that's not actually what you intend.

Delete items from libraries in Final Cut Pro for Mac - Apple Support

I dont understand why apple cant just make a button "Delete unused clips"

You can set the browser to Unused, select all and press Command-Delete. Any clips that were really not used (not even a portion) will be deleted.

As terryb said, be careful not to delete anything you may need (but FCP is smart enough not to delete stuff that is in use).

Legacy FCP could do what you're asking, but it is simply not in FCP 10's wheelhouse.