Remove bundled sphinx-automodapi

[astrofrog]

As discussed in #338, we agreed at the NY meeting that we should split sphinx code out into standalone packages. This was already done for sphinx-automodapi but we should unbundle it and instead make it a dependency of the sphinx-astropy metapackage (once this exists).

In fact, we should check if the remaining sphinx code which is not generically useful could just be moved to sphinx-astropy (which then wouldn't just be a meta-package)