ELF: Remove <Arch>ELFReader.h. NFC.

<Arch>ELFReader.h contains only a few typedefs. The typedefs are used
only by one class in <Arch>TargetHandler.h. Thus, the headers don't
worth to be independent files.

Since MipsELFReader.h contains code other than the boilerplate, I didn't
touch that file in this patch.

llvm-svn: 234056