[ELF] Remove is64bits() and isLittlEndian().

ELFLinkingContext had these two functions, which is really not needed since
the Writer uses a llvm::object template composed of Endianness, Alignment,
Is32bit/64bit. We could just use that and not duplicate functionality.

No Change In Functionality.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221523 91177308-0d34-0410-b5e6-96231b3b80d8