Add an API to remove unused toolchains

[pietroalbini]

There should be an API to remove unused toolchains. Possible implementation idea:

let current = Toolchain::Dist { name: "nightly-1970-01-01".into() };
for toolchain in workspace.installed_toolchains()? {
    if toolchain != current {
        toolchain.remove()?;
    }
}

[pietroalbini]

Fixed on master.