Revert "control/controlclient: use "getprop net.hostname" for Android hostname"

This reverts commit afb9c6a6ab.

Doesn't work. See:

    https://github.com/tailscale/tailscale/issues/409#issuecomment-635241550

Looks pretty dire:

    https://medium.com/capital-one-tech/how-to-get-an-android-device-nickname-d5eab12f4ced

Updates #409

control/controlclient/direct.go

@@ -18,9 +18,7 @@ import (
 	"net/http"
 	"net/url"
 	"os"
-	"os/exec"
 	"reflect"
-	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -161,13 +159,7 @@ func NewDirect(opts Options) (*Direct, error) {
 }
 
 func NewHostinfo() *tailcfg.Hostinfo {
-	var hostname string
-	if runtime.GOOS == "android" {
-		hostname = androidHostname()
-	}
-	if hostname == "" {
-		hostname, _ = os.Hostname()
-	}
+	hostname, _ := os.Hostname()
 	return &tailcfg.Hostinfo{
 		IPNVersion: version.LONG,
 		Hostname:   hostname,
@@ -175,20 +167,6 @@ func NewHostinfo() *tailcfg.Hostinfo {
 	}
 }
 
-var androidHostnameOnce struct {
-	_ structs.Incomparable
-	sync.Once
-	v string
-}
-
-func androidHostname() string {
-	androidHostnameOnce.Do(func() {
-		out, _ := exec.Command("getprop", "net.hostname").Output()
-		androidHostnameOnce.v = strings.TrimSpace(string(out))
-	})
-	return androidHostnameOnce.v
-}
-
 // SetHostinfo clones the provided Hostinfo and remembers it for the
 // next update. It reports whether the Hostinfo has changed.
 func (c *Direct) SetHostinfo(hi *tailcfg.Hostinfo) bool {