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.

src/bin/cargo.rs

@@ -51,7 +51,6 @@ Options:
 
 Some common cargo commands are (see all commands with --list):
     build       Compile the current project
-    check       Analyze the current project and report errors, but don't build object files
     clean       Remove the target directory
     doc         Build this project's and its dependencies' documentation
     new         Create a new cargo project
@@ -76,7 +75,6 @@ macro_rules! each_subcommand{
     ($mac:ident) => {
         $mac!(bench);
         $mac!(build);
-        $mac!(check);
         $mac!(clean);
         $mac!(doc);
         $mac!(fetch);