I have 2 android mobiles and now I'm not using one of them.Is it possible to unlink the mobile which I'm not using from the Google play store?
I would preform a factory reset. This will erase the Google account from the phone and it will erase all of your user data. This way if it gets lost/stolen or you want to give it away it is good to go!
To do a factory reset it is basically the same for all devices:
Settings Menu --> Privacy --> Factory Reset --> Confirm...