value_lexicon: Fix build with abseil LTS 2023-08-02

[acelyc111]

Fix the build error like:

In file included from /usr/local/include/absl/hash/hash.h:85,
                 from /home/xxx/dev/s2geometry/src/s2/s2point.h:24,
                 from /home/xxx/dev/s2geometry/src/s2/value_lexicon_test.cc:28:
/usr/local/include/absl/hash/internal/hash.h:247:23: error: reference to 'hash_internal' is ambiguous
  247 |   using is_hashable = absl::hash_internal::is_hashable<T>;
      |                       ^~~~
In file included from /usr/local/include/absl/hash/internal/hash.h:49,
                 from /usr/local/include/absl/hash/hash.h:85,
                 from /home/xxx/dev/s2geometry/src/s2/s2point.h:24,
                 from /home/xxx/dev/s2geometry/src/s2/value_lexicon_test.cc:28:
/usr/local/include/absl/hash/internal/city.h:57:11: note: candidates are: 'namespace absl::lts_20230802::hash_internal { }'
   57 | namespace hash_internal {
      |           ^~~~~~~~~~~~~
In file included from /home/xxx/dev/s2geometry/src/s2/value_lexicon_test.cc:18:
/home/xxx/dev/s2geometry/src/s2/value_lexicon.h:33:11: note:                 'namespace absl::hash_internal { }'
   33 | namespace hash_internal {
      |           ^~~~~~~~~~~~~

Because absl::hash_internal::Hash is not used in this file, it's safe to remove it.

[jmr]

I'm sure this was added because something was failing, but it's obviously wrong and we shouldn't be depending on abseil internals. I guess we can play whack-a-bug.