Revert "[flang][driver] Allow main program to be in an archive"

This reverts commit 876df74d.

My understanding (based on https://reviews.llvm.org/D149429) is that
this patch has caused all of Flang's buildbots to fail. I'm not really
able to verify 100% as the buildbot UI is incredibly slow ATM. I am
reverting either way so that we can discuss the right solution offline.