[llvm-objcopy] - Remove dead code. NFCI.

DecompressedSection can only be created if --decompress-debug-sections is specified.
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/ELF/ELFObjcopy.cpp#L492

If it is specified when !zlib::isAvailable(), we error out early when parsing the options:
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/CopyConfig.cpp#L657

What means the code I am removing in this patch is dead.

Differential revision: https://reviews.llvm.org/D59017

llvm-svn: 355505