Run the following one line command and then clear your browser cache (depending on the browser you may need to open a new tab or restart the browser):
sed -Ezi.bak "s/(function\(orig_cmd\) \{)/\1\n\torig_cmd\(\);\n\treturn;/g" /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js && systemctl restart pveproxy.service
Source: https://johnscs.com/remove-proxmox51-subscription-notice/