Remove unused import of pydot.

theano/d3viz/__init__.py

 from theano.d3viz.d3viz import d3viz, d3write
-
-has_requirements = False
-try:
-    # pydot-ng is a fork of pydot that is better maintained
-    import pydot_ng as pd
-except ImportError:
-    # fall back on pydot if necessary
-    import pydot as pd
-if pd.find_graphviz():
-    has_requirements = True