samples: Remove unused CardMaker import from procedural-cube sample

Backport of 5d465be01e9ac5a8aad9cb689af822f60682b7e9

samples/procedural-cube/main.py

@@ -17,7 +17,6 @@ from panda3d.core import GeomVertexFormat, GeomVertexData
 from panda3d.core import Geom, GeomTriangles, GeomVertexWriter
 from panda3d.core import Texture, GeomNode
 from panda3d.core import PerspectiveLens
-from panda3d.core import CardMaker
 from panda3d.core import Light, Spotlight
 from panda3d.core import TextNode
 from panda3d.core import LVector3