[ELF] Remove class declarations that do nothing.

These classes are declared in a .cpp file but not used in the same compliation
unit. They seems to have been copy-and-pasted from ELFReader.h.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204988 91177308-0d34-0410-b5e6-96231b3b80d8