Revert part of r193291, restoring the deletion of loaded objects.

Without this, customers of the MCJIT were leaking memory like crazy.

It's not really clear what the *right* memory management is here, so I'm
not trying to add lots of tests or other logic, just trying to get us
back to a better baseline. I'll follow up on the original commit to
figure out the right path forward.

llvm-svn: 193323