Hi.
Its actually not a delete but you can set material properties to null
#get surface count
var surf_count = $MeshInstance.mesh.get_surface_count()
for i in range(surf_count):
#for each of the surfaces, set material to null
$MeshInstance.mesh.surface_set_material(i, null)
#also set the override material to null
$MeshInstance.material_override = null
this will leave the mesh unrenderable ... it will not be shaded after that