xen: remove CONFIG_XEN_COMPAT, purge Xen 3.0 compatibility
This overlaps the purpose of XEN_INTERFACE_VERSION. Remove Xen 3.0.2
compatibility. XEN_INTERFACE_VERSION has compatibility to Xen 3.2.8
enabled. As Xen 3.3 was released almost 15 years ago, it seems unlikely
anyone hasn't updated.
Reviewed by: royger