How can I delete this account or merge it into a different account?
First, you will need to edit any transaction that affected the account to move them to another account.
Once, this is done you can then delete the account
If you have a lot of transactions, then you could look into using the Batch Update feature but this might not be as simple as it appears
Cash at bank is a control account. You cannot delete it. It appears automatically when you have any bank accounts in use. It summarizes all bank accounts.
There is no “merge bank accounts” capability but it’s not really needed since you can move all receipts / payments / inter-account transfers from one bank account to another using batch operations as @Joe91 mentions.