Hi Maike,
you can delete the reservations. it will not have any impact on any other object. resarvations is just to pass the information about the quantity requirements. other than this they will not have any other use or impact on the business.
as the standard behaviour system will craete Resarvations for all the components of the BOM. i think you can remove the bulk material from your BOM it will not have any affects on your process.
Thanks&Regards,
Venkat.