[elf] Remove one copy of the section merging code

Summary:
Besides being superfluous, this double merging was actually wrong and
causing some sections to be added twice. The reason for that was that
the code assumes section IDs are unique in the section list, but this is
only true if all sections in the list come from the same object file.

Reviewers: fjricci, jankratochvil

Subscribers: emaste, lldb-commits, arichardson

Differential Revision: https://reviews.llvm.org/D44157

llvm-svn: 327123