Removed reStructuredText ftplugin

@terminalmage this change ok with you?

It feels off to me to have a rST ftplugin along with unrelated Vim files. (This isn't the first place I'd look to track down where a particular rST setting was coming from.)

It might make sense to have a "salt-docs-vim" repo if we had a little more settings/content to fill it out. Extra rST keywords, and a ``path`` addition perhaps?