Switch to native mdbook admonitions

[avivace]

mdbook 0.5.x now ships admonitions:

https://rust-lang.github.io/mdBook/format/markdown.html#admonitions

Once we upgrade to 0.5.x (#641) we can drop the custom code for admonitions and use the build it ones.

[mtthgn]

@avivace, as part of the 0.5 upgrade, I was looking into this as well. The built in implementation doesn't support custom headings, which pandocs is using on nearly every admonition. Based on the closing of rust-lang/mdBook#2966, it doesn't seem like there's much interest in further customization when preprocessors are an option.

In my opinion, we should close this one and continue to use the existing admonition functionality.