Terrain>Surfaces>Edit>Delete Surface Features
You can delete features on each inner surface by selecting the surface at the top of the panel
You can delete all surface features at one time. Please note that it deletes all surface feature not just one type or layer. I would recommend exporting the surface feature and saving a new file before doing this in case you need to go back to this point.
Can get this same delete all function through Terrain>Delete>Surface Features
You can also left click to select then hold down shift to select the next feature and delete the features you more selectively. Once all are selected can right click>delete or go to Edit>Delete
Another way to select a group of features is the select icon.
This tool is also under Edit>Select Rectangle or Select Polygon
Then you click and drag to make a rectangle around the features you want to delete.