remove mention to %pylab [backport to 1.4.2-doc]

Remove references of %pylab and/or warn not to use. 

In image tutorial it was completely unused later. And of course people on Stack Overflow
always appear to be able to find the most deeply buried documentation to give bad advices.


It would be good to have this link to something, so that a user who wants to follow this advice can immediately see how to do so.