[llvm-objcopy] Remove unused field from Object

The class Object contains std::shared_ptr<MemoryBuffer> OwnedData
which is not used anywhere. Besides avoiding two stage initialization 
the motivation to remove it comes from the plan to add (currently missing) support 
for static libraries.
NFC.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D47855

llvm-svn: 334217