Remove outdated advice for Vim plugins (#600)

Author: amarz45

README.md

@@ -262,8 +262,6 @@ The following options control which syntax extensions will be turned on. They ar
 
     Highlight TOML front matter as used by [Hugo](https://gohugo.io/content/front-matter/).
 
-    TOML syntax highlight requires [vim-toml](https://github.com/cespare/vim-toml).
-
         let g:vim_markdown_toml_frontmatter = 1
 
 #### JSON Front Matter
@@ -272,8 +270,6 @@ The following options control which syntax extensions will be turned on. They ar
 
     Highlight JSON front matter as used by [Hugo](https://gohugo.io/content/front-matter/).
 
-    JSON syntax highlight requires [vim-json](https://github.com/elzr/vim-json).
-
         let g:vim_markdown_json_frontmatter = 1
 
 #### Strikethrough