[lib/ObjectYAML] - Cleanup the private interface of ELFState<ELFT>. NFCI.

In D62809 I accidentally added "ELFState<ELFT> &State" as the
first parameter to two methods. There is no reason for having that.
I removed this argument and also moved finalizeStrings declaration to
remove an excessive 'private:' tag.

Differential revision: https://reviews.llvm.org/D67157

llvm-svn: 371033