[Libomptarget] Do not check for valid binaries twice.

The only RTLs that get added to the `UsedRTLs` list have already been
checked is they were valid binaries. We shouldn't need to do this again
when we unregister all the used binaries as they wouldn't have been used
if they were invalid anyway. Let me know if I'm incorrect in this
assumption.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D131443