[Offload][NFC] Remove 'libomptarget' message helpers (#92581)

Summary:
This isn't `libomptarget` anymore, and these messages were always
unnecessary because no other project uses these prefixed messages. The
effect of this is that no longer will the logs have `LIBOMPTARGET --` in
front of everything. We have a message stating when we start building
the offload project so it'll still be trivial to find.