Once an asset has been allocated a design, there doesn't seem to be a way of removing the design.
The only options seem to be add a new design or overwrite with another design, not remove current design and go back to it's initial design-less state.
If you hit "Clear" on the System Defined Design where the design is applied and click Commit, Matrix should remove the design from that asset (and any assets that received that design via inheritence).
However, assets will always inherit designs from their parent/ancestors, so if you want to try and set an asset lower down the tree to have no design, that won't work. You could create a blank design though (with just a body design area) and use that instead.