Remove boot password from the command line
vdc cryptfs changepw default current_password default_password


does the trick. Replace current_password with the real current password. 

I believe the new password specified is irrelevant, as the DEFAULT_PASSWORD constant is used anyway. However, the current password seems to be required, atleast when using hardware-based encryption.
(see https://github.com/LineageOS/android_system_vold/blob/61a3249612495e7a5b2a1950859e9eb43dfc4559/cryptfs.cpp#L2858)