Remove code deprecated in 0.3 and 0.4

My understanding of the deprecation process is that we are deprecating functions/classes/methods in one stable version and removing it in the next. This PR removes code marked as deprecated in 0.3 and 0.4 since it should be removed before 1.0. I was thinking it would make sense to do this sooner rather than later so that packages that test against 1.0 won't see these functions disappear at the last minute before the 1.0 release. We shouldn't backport this to v0.4.x though.

@embray - there is actually a fair bit of deprecated code in astropy.io.fits - shall we remove it soon so that it doesn't get included in 1.0?

cc @eteq @embray @mdboom @taldcroft @nhmc @aconley (since this includes changes in one of your subpackages)
For the Planck 13 cosmology (flat, w=-1), Ok, de_density_scale, and w are redshift independent.  So z_at_value can't invert the function.  If you want to test those three, you will need to create a more 'interesting' cosmology.