Remove old test code from renderables/gradient.py

This isn't currently covered in unit tests but should be at some point?
Either way, having a test in dead code in the library doesn't help much any
more.

src/textual/renderables/gradient.py

@@ -36,9 +36,3 @@ class VerticalGradient:
                 ),
             )
             yield Segment(f"{width * ' '}\n", line_color)
-
-
-if __name__ == "__main__":
-    from rich import print
-
-    print(VerticalGradient("red", "blue"))