[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.

libcxx/include/__ranges/data.h

@@ -68,7 +68,7 @@ namespace __data {
 } // end namespace __data
 
 inline namespace __cpo {
-  inline constexpr const auto data = __data::__fn{};
+  inline constexpr auto data = __data::__fn{};
 } // namespace __cpo
 } // namespace ranges
 

libcxx/include/__ranges/size.h

@@ -114,7 +114,7 @@ namespace __ssize {
 }
 
 inline namespace __cpo {
-  inline constexpr const auto ssize = __ssize::__fn{};
+  inline constexpr auto ssize = __ssize::__fn{};
 } // namespace __cpo
 } // namespace ranges