Clean up os(WASI) in Subprocess.swift

Author: MaxDesiatov

stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift

@@ -25,9 +25,6 @@ import WinSDK
 #if !os(WASI)
 // No signals support on WASI yet, see https://github.com/WebAssembly/WASI/issues/166.
 internal func _signalToString(_ signal: Int) -> String {
-#if os(WASI)
-  return "unsupported"
-#else
   switch CInt(signal) {
   case SIGILL:  return "SIGILL"
   case SIGABRT: return "SIGABRT"
@@ -40,7 +37,6 @@ internal func _signalToString(_ signal: Int) -> String {
 #endif
   default:      return "SIG???? (\(signal))"
   }
-#endif // os(WASI)
 }
 #endif