Now documented here:
https://winscp.net/eng/docs/master_password#reset

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Old answer:
Remove Configuration\Security key in configuration. If you are using Registry storage for configuration, remove

[HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\Configuration\Security]

Also remove all Password values for sessions.

Make sure WinSCP is shut down, when you are modifying the configuration.

For the life of me I can't remember what the master password is and there seems to be no option to reset or remove it without knowing it.

Obviously the master password is used to encrypt the FTP passwords, so resetting or removing it would clear those. That's fine, I can enter them again after setting a new password. I just miss the convenience of not having to enter them every time.