Respect form value for register_date on partipant form

Respect form value for register_date on partipant form

The backoffice event registration form has a register_date field. In credit card mode it
does an awful lot of handling which is mostly for legacy code reasons but the net result is to ignore it
& set it to now. However, the field defaults to now & if someone actually changed it they would expect their change to
be respected. This removes that handling & just uses the field value entered. The field is always present from my
digging