Hi,
If you want to remove the static 0.0.0.0/0 default route, you need to delete it completely "delete routing-options static route 0.0.0.0/0". Looks like you deleted using "delete routing-options static route 0.0.0.0/0 discard" which will remove only "discard" keyword and that's why you get that commit error.
Also as stated earlier, if you are expected the 10.10.10.0/29 subnet in the Core router to be resolved via this default 0.0.0.0/0 route, then you need to configure a valid next-hop for it. I am not sure about your design, but if you want to reach the BGP, you can point this default route to the BGP peer on Core. Something like this:
set routing-options static route 0.0.0.0/0 next-hop <ip address of the interface pointing towards BGP peer>
Thanks