drop support for building 32bit apple docs

Rust's RFC 2837 demoted the i686-apple-darwin target to Tier 3, so that
target will not be downloadable anymore through rustup starting from
Rust 1.42.0. The change will also land on nightly soon, and that will
prevent docs.rs from updating the rustwide toolchain.

This removes i686-apple-darwin from the list of targets we build.

Author: pietroalbini

src/docbuilder/rustwide_builder.rs

@@ -20,7 +20,6 @@ static USER_AGENT: &str = "docs.rs builder (https://github.com/rust-lang/docs.rs
 static DEFAULT_RUSTWIDE_WORKSPACE: &str = ".rustwide";
 
 static TARGETS: &[&str] = &[
-    "i686-apple-darwin",
     "i686-pc-windows-msvc",
     "i686-unknown-linux-gnu",
     "x86_64-apple-darwin",