The clearbanned RPC clears all banned IPs.
clearbanned none
*bitcoin-cli help clearbanned
clearbanned
Clear all banned IPs.
Examples:
> bitcoin-cli clearbanned
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "clearbanned", "params": [] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/