*(global): fixed service implementation for OpenWrt

We now use a procd init script for OpenWrt just like it's recommended in
the documentation. The service is automatically enabled on the install
command.

✅ Closes: https://github.com/AdguardTeam/AdGuardHome/issues/1386

home/control_update.go

@@ -152,15 +152,6 @@ type updateInfo struct {
 	newBinName       string // Full path to the new executable file
 }
 
-// Return TRUE if file exists
-func fileExists(fn string) bool {
-	_, err := os.Stat(fn)
-	if err != nil {
-		return false
-	}
-	return true
-}
-
 // Fill in updateInfo object
 func getUpdateInfo(jsonData []byte) (*updateInfo, error) {
 	var u updateInfo