[sphinx] Fixed warnings

doc/conf.py

@@ -26,16 +26,8 @@ extensions = [
     'sphinx.ext.napoleon',
     'sphinx.ext.viewcode',
     'sphinx.ext.mathjax',
-    #'matplotlib.sphinxext.plot_directive'
     ]
 
-import sphinx
-if sphinx.__version__ >= "1.4.1":
-    extensions.append('sphinx.ext.imgmath')
-    imgmath_image_format = 'svg'
-else:
-    extensions.append('sphinx.ext.pngmath')
-
 # Be picky about warnings
 nitpicky = True