ELF: Remove partial class definitions of <Arch>LinkingContexts.

What we are doing in ELFTarget.h was dubious. In the file, we define
partial classes of <Arch>LinkingContexts to declare only static member
functions. We have different (complete) class definitions in other files.
They would conflict if they exist in the same compilation unit (because
the ones defined in ELFTarget.h has only static member functions).
I don't think this was valid C++.

http://reviews.llvm.org/D8797

llvm-svn: 234039