By coincidence I stumbled upon satoshi's reasoning why you can't remove a private key from Bitcoin Core:
If you were able to delete a bitcoin address and someone sent to it, the money would be lost. They're only about 500 bytes.
This quote is from when core didn't use HD wallets.
Today, deleting a private key and the associated address shouldn't be a problem.
It would be kind of blacklisted in the wallet.dat file (i.e. not showing / scanning this address). The seed however would still reproduce the private key of this address.
Technically this could be feasible, but probably unnecessary and problematic.
