lnrpc/autopilotrpc: remove unused "os" import

This would cause the build to fail since the import is not used. It was
not detected since the file is under a build flag.

lnrpc/autopilotrpc/autopilot_server.go

@@ -5,7 +5,6 @@ package autopilotrpc
 import (
 	"context"
 	"encoding/hex"
-	"os"
 	"sync/atomic"
 
 	"github.com/btcsuite/btcd/btcec"