Use the default Markdown Sublime syntax

[sharkdp]

Apparently, the default Markdown syntax from sublimehq/Packages is quite good (see trishume/syntect#156 (comment)).

I was reluctant to move away from sublime-markdown-extended because I thought that fenced code blocks would not be highlighted properly. However, this was just me using whitespace between opening fences ``` and the language definition (see sublimehq/Packages#1615 and sublimehq/Packages#1616). We should consider to move, should the upstream ticket be resolved.