IPDoctest replaces the normal doctest plugin. Previously we did this by
initializing the builtin plugins but skipping the doctest plugin.
However, if the user has a noserc file with 'with-doctest=1' or the
environment variable 'NOSE_WITH_DOCTEST', then nose will try and
initialize the doctest plugin when it isn't there, and barf. This
commit defers the removal of the doctest plugin to the configuration
stage, so doctest can be enabled before it is thrown away by us.