I mean, I don't want that group to appears in whatsapp what-so-ever
If this is made possible by WhatsApp, how are you suppose to send message to that group? If you cannot send the message, then what is point of having that group?
You may want to check the WhatsApp FAQ but AFAIK there is no mention of Hiding the group in Whatsapp.
If you do not want to send the messages from that group, you can simply exit the group
Another option is to mute the group. You will still receive messages sent to the group, but your phone will not vibrate or make noise when they are received.
If your only intention is to avoid "other people" from knowing which WhatsApp group you belong to, then you can use Locking apps to achieve it.
If not, is there any app that can easily hide (ie, remove a group from
Android contacts) and unhide whenever you want?
That group is specific to WhatsApp and is not known to other parts of Android system as well as to other apps.
It is very unlikely that WhatsApp will allow other app to operate(hide/unhide) on the User data belonging to WhatsApp. Not possible AFAIK.