Incident Layout 0 is the default OOTB layout as far as I am aware. As long as you have moved allt he roles onto your own design I see no problem deleting it - if you can!
I have the same problem that although the administrator role now belongs to a new layout it also belongs to the Incident Layout 0 too. I did some tests and it looks like its not the same administrator role - almost like there are two administrator roles with the same display name.
I've just tried to remove the layout and as suspected you will get an error -
- Unhandled system exception: Layout Incident Layout 0 referenced in role "ServiceOwner". Ref. found in: Workspaces[3].jsonConfig.myItemsConfig.LayoutName role "Admin". Ref. found in: Workspaces[121].jsonConfig.myItemsConfig.LayoutName object "ServiceReqTemplate[09B9CA5C5A3A4F40BCEC7F5369FF8FBF]". Ref. found in: Data[Cannot Access Drive X].BusinessObjectLayoutName object "ServiceReqTemplate[10375B9F10F3420E92278A3EE5F4C6D9]". Ref. found in: Data[Voicemail Problem].BusinessObjectLayoutName object "ServiceReqTemplate[4246EF15E7BA4A40BFC07C9BC9DFE336]". Ref. found in: Data[VPN Connection Failure].BusinessObjectLayoutName and 12 more references
This error suggest that the ServiceOwner role is also assigned to the 'Incident Layout 0' layout. Going over to the ServiceOwner role in the page layouts workspace I found incident was not even setup so I tried adding the incident object workspace to the ServiceOwner and now also have serviceowner assigned to both Incident Layout 0 and the redesigned layout.
If I then try to delete the layout I get -
- Unhandled system exception: Layout Incident Layout 0 referenced in role "Admin". Ref. found in: Workspaces[121].jsonConfig.myItemsConfig.LayoutName role "ServiceOwner". Ref. found in: Workspaces[23].jsonConfig.layout object "ServiceReqTemplate[09B9CA5C5A3A4F40BCEC7F5369FF8FBF]". Ref. found in: Data[Cannot Access Drive X].BusinessObjectLayoutName object "ServiceReqTemplate[10375B9F10F3420E92278A3EE5F4C6D9]". Ref. found in: Data[Voicemail Problem].BusinessObjectLayoutName object "ServiceReqTemplate[4246EF15E7BA4A40BFC07C9BC9DFE336]". Ref. found in: Data[VPN Connection Failure].BusinessObjectLayoutName and 13 more references
I changed the admin over to Incident Layout 0 too and I now have this setup -

It looks like a bug to me and you will struggle to remove the role.
Raised with HEAT on Incident ref - 30216317