[Libomptarget] Remove extra cache for offloading entries (#77012)

Summary:
The offloading entries right now are assumed to be baked into the binary
itself, and thus always valid whenever the library is executing. This
means that we don't need to copy them to additional storage and can
instead simply pass around references to it.

This is not likely to change in the expected operation of the OpenMP
library. Additionally, the indirection for the offload entry struct is
simply two pointers, so moving it by value is trivial.