[libc][test] Remove more unused nan variables (#155925)

These are redefined/shadowed by the if constexpr (FPBits::FRACTION_LEN -
1 >= 5) case below.

Added by https://github.com/llvm/llvm-project/pull/155569. I missed
these ones in #155894.

GitOrigin-RevId: 3ea7956fd326defab6d1b9a97d9bbd80b5ca3505