Remove u.eig etc

This may be a controversial PR and therefore I assign this to @gavinsimpson 

As long as we have documented `cca.object` (and that is from **vegan** 1.6-10 in Sep, 2005) we have told that there is "no guarantee" to keep `u.eig`, `v.eig` and `wa.eig` items. Really removing them after nine years of silent life can be abrupt. We changed to more decisive language in 2013 ("will be removed"), but that threat was never in the release versions. We may consider delaying this move to 2.2-1. 

No **vegan** function uses these items, but I have no idea about other packages depending or using **vegan**. There is no informative deprecation mechanism for result objects so that there is no graceful way of deprecation. It seems that we break other things with the major release (2.2-0) -- at least with **vegan3d** split -- so that this could be a suitable slot for this breakage as well. However, I leave this to @gavinsimpson to decide.