ci: remove 32-bit Apple targets
This PR drops the `i686-apple` and `dist-i686-apple` CI builders, as well as removing the `armv7-apple-ios`, `armv7s-apple-ios` and `i386-apple-ios` targets from the `x86_64-apple` CI builder.
The change was approved in [RFC 2837](https://github.com/rust-lang/rfcs/pull/2837), and it should land in Rust 1.42 stable (so this cycle).
r? @alexcrichton