Remove inheritance_diagram.py.

Since b107f0a, we are using inheritance_diagram.py provided
by Sphinx ≥ 0.6.

Author: mitya57

doc/sphinxext/README.txt

@@ -5,13 +5,10 @@
 We've copied these sphinx extensions over from nipy-core.  Any edits
 should be done upstream in nipy-core, not here in nipype!
 
-Thesea are a few sphinx extensions we are using to build the nipy
+These a are a few sphinx extensions we are using to build the nipy
 documentation.  In this file we list where they each come from, since we intend
 to always push back upstream any modifications or improvements we make to them.
 
-* From matploltlib:
-  * inheritance_diagram.py
-
 * From numpy:
   * numpy_ext