rustdoc should not automatically insert feature gates for testing code snippets in plain markdown files

[huonw]

Currently running rustdoc --test foo.md will automatically insert

#[ feature(macro_rules, globs, struct_variant, managed_boxes) ];

into every snippet. This is a stopgap measure added by #12747 to easy migration from the old extract-tests.py driver to the new rustdoc --test one.

Fixing this would preferably involve removing as many of the uses of feature gated things as possible from the tutorial, manual and guides.

[alexcrichton]

triage: Our current plans are to start cracking down on #[feature] usage, and features such as the ones mentioned are all slated for inclusion into the standard stable distribution, so I'm going to close this for now. It should be noted that today rustdoc doesn't actually inject any feature-gate enablings.