Remove unreachable customGroup section in online event receipt

Remove unreachable customGroup section in online event receipt

After digging into https://github.com/civicrm/civicrm-core/pull/18697 I came to the
conclusion that currently both the online & offline event receipts have
sections for custom profiles and custom data (marked as customGroup).

The profiles are only assigned from the online flow and the data only from the offline
flow so in each receipt template one is unused.

I agree with the discussion on https://lab.civicrm.org/dev/core/-/issues/2076
that suggests that since every event has (potentially) assigned profiles
we could use those in both scenarios and never use the generic 'customGroup'.
The situation is less clear for contributions (where there is no profile
if there is no contribution page). Thus, I am inclined
to remove customGroup from the offline template too, after fixing to
include the profiles. However, there are additional
steps to get to the point where that is doable - at this stage I think
we CAN remove it from the online template because I cannot find anywhere
in the code where customGroup is assigned in that flow, nor logically
think of any reason other than copy and paste for it