Remove old docs when upgrading sagenb

Imported from <a href="http://trac.sagemath.org/sage_trac/ticket/13405">trac #13405</a>.



Reported by: jdemeyer

Component: build

CC: 

Report Upstream: N/A

Authors: Jeroen Demeyer, Keshav Kini

Dependencies: 

Work issues: 

Reviewers: John Palmieri

Merged in: sage-5.3.rc1

Attachments: <ol><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/13405/sagenb-0.9.2.diff">sagenb-0.9.2.diff: </a> by jdemeyer at 20120829T10:13:36</li></ol>