remove obsolete code

nifty2go/plotting/plotter/plotter_base.py

@@ -24,8 +24,6 @@ if plotly is not None and 'IPython' in sys.modules:
 
 class PlotterBase(with_metaclass(abc.ABCMeta, type('NewBase', (object,), {}))):
     def __init__(self, interactive=False, path='plot.html', title=""):
-        if plotly is None:
-            raise ImportError("The module plotly is needed but not available.")
         self.interactive = interactive
         self.path = path