Remove unused file, ExecutionEngine/MCJIT/ObjectBuffer.h

This header also wasn't self contained/modular - but with no users, it
didn't seem worth fixing because it'd break so easily again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328565 91177308-0d34-0410-b5e6-96231b3b80d8

Author: dwblaikie