Remove version{4,5,6,7}.txt

These files describe changes for old vim releases with much really outdated information ("This file is important for everybody upgrading from Vim 3.0").

They show up as noise when grepping for features (see e.g. https://github.com/neovim/neovim/pull/1081) and should probably be removed since they are only noise for users when e.g. mentioning it in help.txt.