'Compiler Features' section overlaps with Unstable Book

[frewsxcv]

The Unstable Book is going to document all the currently unstable features. Is it worth keeping the content in this section around? https://doc.rust-lang.org/nightly/reference/attributes.html#compiler-features

Relevant issue: #9

[steveklabnik]

I don't think it is. There was nowhere else to put this before, but now there is.