DOC: remove image_registration from labs docs
DOC: remove image_registration from labs docs

I remoed the image_registration.rst file a few commits back because it
was badly out of date, and wasn't any longer in the labs package.  I
forgot to remove it from the toctree of labs/index.rst.

bthirion/nipy,arokem/nipy,alexis-roche/nipy,alexis-roche/nipy,arokem/nipy,alexis-roche/register,alexis-roche/niseg,alexis-roche/nipy,bthirion/nipy,alexis-roche/nipy,alexis-roche/niseg,bthirion/nipy,nipy/nireg,alexis-roche/register,nipy/nipy-labs,alexis-roche/nireg,nipy/nipy-labs,nipy/nireg,arokem/nipy,bthirion/nipy,arokem/nipy,alexis-roche/nireg,alexis-roche/register