Remove vim-markdown plugin.

Remove the vim-markdown plugin from vimrc file. The vim-markdown plugin
provided it's own Markdown syntax highlight rules. In addition, it
provided it's own filetype on which those syntax rules were applied.
Lastly, it mapped popular Markdown file extensions to the custom filetype.

The syntax highlighting rules, and custom Markdown filetype, are
unnecessary. Vim comes with support, via it's runtime files, for Markdown
files. We only need to map files with *.md as their file extension as
Markdown files to properly apply Vim's existing syntax highlighting rules.