editors/vim-colorschemes-legacy: Remove RUN_DEPENDS, change install dir

As noted by cy, setting RUN_DEPENDS=vim means that any other vim flavor
gets uninstalled when this port is installed. Nothing in this port
actually depends on Vim at all; it should work equally well for Neovim
or any Vim flavor. It makes most sense to just drop the RUN_DEPENDS
altogether.

While here, change the install dir to PREFIX/share/vim/vimfiles. The
previous dir, .../vim/vim90, is the private runtime library, and
vimfiles has over time become the site extension to the runtime library.