MNT: remove reference to deleted example

examples/tests/backend_driver.py

@@ -127,7 +127,6 @@
     'barcode_demo.py',
     'boxplot_demo.py',
     'broken_barh.py',
-    'clippedline.py',
     'cohere_demo.py',
     'color_by_yvalue.py',
     'color_demo.py',