i have to delete my electrum wallet,
because i am selling the computer on which it is saved.
Are you going to completely wipe your computer's hard drive before selling it or simply uninstalling the Electrum software? If it's the latter then make sure to delete the folder containing your wallet file even if it's encrypted (always backup your wallets seeds even if you think you will never use/need them again).
I think i can't access the wallet anymore, when i delete the system-data...maybe i'm wrong...
If you've saved the seed (12 word recovery phrase) then you can use it to restore your wallet on any other device any time you want.
To recover your wallet, install Electrum -> Create New Wallet -> Standard Wallet -> I already have a seed