Remove dead code.

Author: Julian

jsonschema/_format.py

@@ -383,11 +383,6 @@ def is_css21_color(instance):
             return True
         return is_css_color_code(instance)
 
-    def is_css3_color(instance):
-        if instance.lower() in webcolors.css3_names_to_hex:
-            return True
-        return is_css_color_code(instance)
-
 
 with suppress(ImportError):
     import jsonpointer