It was added in ddee45e1d7 when SIGPIPE was controlled with an
attribute on `fn main()` which meant it could also be combined with
`#[rustc_main]`:
#[unix_sigpipe = "sig_dfl"]
#[rustc_main]
fn rustc_main() {
It stopped being needed cde0cde151 when `#[unix_sigpipe = "..."]` was
replaced by `-Zon-broken-pipe=...`. And it will not be needed when
`-Zon-broken-pipe=...` is replaced by an Externally Implementable Item.
Let's remove this test.
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