[llvm-objcopy] [COFF] Remove pointless Reader/Writer base classes. NFC.

These were copied as part of the original design from the ELF
backend, but aren't necessary at the moment.

Differential Revision: https://reviews.llvm.org/D56431

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350996 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed