applehv: Remove unneeded cmd.ExtraFiles assignment

`applehv.Start()` has this line of code:
```
cmd.ExtraFiles = []*os.File{ioEater, ioEater, ioEater}
```
whose purpose is not clear.

The intent may have been to redirect stdin/stdout/stderr to /dev/null in
the child process.
This should be done by setting cmd.Stdin/cmd.Stdout/cmd/Stderr to nil,
which is the case by default.

The way it's done could also cause issues as
`Vfkit.VirtualMachine.Cmd()` sometimes adds files it needs to keep open
to `ExtraFiles`, so at the very least this should be an `append()`

This commit removes this code.

[NO NEW TESTS NEEDED]

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>

pkg/machine/applehv/machine.go

@@ -572,12 +572,6 @@ func (m *MacMachine) Start(name string, opts machine.StartOptions) error {
 		return machine.ErrVMAlreadyRunning
 	}
 
-	ioEater, err := os.OpenFile(os.DevNull, os.O_WRONLY, 0755)
-	if err != nil {
-		return err
-	}
-	defer ioEater.Close()
-
 	// TODO handle returns from startHostNetworking
 	forwardSock, forwardState, err := m.startHostNetworking()
 	if err != nil {
@@ -641,8 +635,6 @@ func (m *MacMachine) Start(name string, opts machine.StartOptions) error {
 		cmd.Args = append(cmd.Args, "--gui") // add command line switch to pop the gui open
 	}
 
-	cmd.ExtraFiles = []*os.File{ioEater, ioEater, ioEater}
-
 	readSocketBaseDir := filepath.Dir(m.ReadySocket.GetPath())
 	if err := os.MkdirAll(readSocketBaseDir, 0755); err != nil {
 		return err