To remove one or more sub-assets from an asset, simply select all the assets you want to remove, and drag them into an empty space in the Project window (or directly to another folder).
Dragging two sub-assets out of their main asset
Just like that, the sub-assets will be extracted and become individual main assets.
SecondaryMission_1 and _2 are now independent assets
Extracting a sub-asset like this is immediate, and doesn't present a confirmation window.
Preserving References
When adding a sub-asset using drag and drop, normally the references to the original asset will break. SubAssets Toolbox has a way to prevent this, and you can find more about it in Patch References.
Deleting sub-assets
If you want to delete a sub-asset and not just unparent it, you can also select it and use right-click > Delete SubAsset(s).
Confirmation for deleting a sub-asset
Alternatively, you can hit Ctrl+Backspace (Windows) or Cmd+Backspace (Mac) as a shorcut.
When deleting a sub-asset this way, Unity will present this warning in the console:
The operation "Delete" cannot be executed because the selection is a root folder.
