Remove the Native file format.

The Native file format was designed to be the fastest on-memory or
on-disk file format for object files. The problem is that no one
is working on that. No LLVM tools can produce object files in
the Native, thus the feature of supporting the format is useless
in the linker.

This patch removes the Native file support. We can add it back
if we really want it in future.

llvm-svn: 234641