[libc++] Remove a stray `const` on ranges::data and ranges::ssize. NFCI.

These are specced as `inline constexpr auto`; the extra `const`
isn't doing anything except being inconsistent with the other CPOs.
Now all the implemented CPOs can be detected by
    git grep 'inline constexpr auto.*fn' ../libcxx/include/
and I think that's beautiful.