Firstly I want to tell you that I am using Bitcoind with RPC commands.
1- Is bitcoin core generate private key with wallet address and save it to server or it generate it when we call function which return wallet private keys ?
2- if bitcoin core server save private keys on server then how i can delete them?
Thanks
If the wallet is unencrypted, then you should be able to enter the wallet.dat file and delete the private keys. You'd have to delete other intricate information specific to that without destroying other information which is why it's a bad idea to do it and could end up losing you all of the bitcoins in tat wallet (as achow101 has suggested).