runtime: remove darwin workaround for process-wide raise

Now that raise on darwin targets the current thread, we can remove
the workaround in dieFromSignal.

Change-Id: I1e468dc05e49403ee0bbe0a3a85e764c81fec4f2
Reviewed-on: https://go-review.googlesource.com/110476
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/signal_unix.go

@@ -440,14 +440,6 @@ func dieFromSignal(sig uint32) {
 	osyield()
 	osyield()
 
-	// On Darwin we may still fail to die, because raise sends the
-	// signal to the whole process rather than just the current thread,
-	// and osyield just sleeps briefly rather than letting all other
-	// threads run. See issue 20315. Sleep longer.
-	if GOOS == "darwin" {
-		usleep(100)
-	}
-
 	// If we are still somehow running, just exit with the wrong status.
 	exit(2)
 }