Auto merge of #3465 - alexcrichton:remove-cargo-check, r=brson
Remove the `cargo check` subcommand from Rust 1.15
This commit removes the `cargo check` subcommand from the `rust-1.15.0` branch
of Cargo. The rationale is explained in rust-lang/rust#38666, but the tl;dr; is
that it's relatively buggy and not ready to ride the trains to stable. This
commit takes as little an invasive as approach as possible by simply removing
the `cargo check` command and associated tests. The backend remains with support
for `cargo check`, but we shouldn't be exercising it in practice.
