I opened your drawing, turned on layer "N Trellis", because it was turned off, now I see that there are 8 objects on the layer "N Trellis" - 2 lines and 6 dimension objects:
AutoCAD treats dimension objects as blocks (unanimous blocks), that is why you cannot remove this layer.
Even if you ERASE these dimension objects, you still need to remove their definitions with PURGE command or you can close and reopen drawing after erasing dimension objects. After PURGE or reopening drawing you will be able to remove "N Trellis" layer from the drawing.
In short:
If you have dimensions in the drawing, AutoCAD treats them as blocks. If you erase these dimension objects their "unanimous definitions" remains in the drawing until you PURGE them or until you save and reopen drawing.