Remove GeomCol

[thomasp85]

GeomCol is a direct duplicate of GeomBar. We should remove it and simply make geom_col() a GeomBar+StatIdentity call

[teunbrand]

Should GeomCol go through some deprecation process? We could have ggproto() throw an error when you attempt to inherit from GeomCol.

[teunbrand]

Closing this issue in favour of broader scope of #5962