remove explicit default

closes  #8449

IPython/core/magics/pylab.py

@@ -42,7 +42,7 b' class PylabMagics(Magics):'
     @skip_doctest
     @line_magic
     @magic_arguments.magic_arguments()
-    @magic_arguments.argument('-l', '--list', default=False, action='store_true',
+    @magic_arguments.argument('-l', '--list', action='store_true',
                               help='Show available matplotlib backends')
     @magic_gui_arg
     def matplotlib(self, line=''):