Remove all smart_holder.h includes and PYBIND11_SMART_HOLDER_TYPE_CASTERS macros.

[rwgk]

Please refer to #1321

Please ignore this PR.

Original description:

The smart_holder.h include and the PYBIND11_SMART_HOLDER_TYPE_CASTERS macros were made obsolete by pybind/pybind11#5257 (merged on Jul 31, 2024).

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[rwgk]

@lanctot I believe these simple changes should work with:

• what you have Google-internally
• practically any commit of the smart_holder branch after [smart_holder] Bake smart_holder functionality into class_ and type_caster_base pybind/pybind11#5257 (merged on Jul 31, 2024)
• pybind11 master after feat: squash-merge smart_holder branch into master pybind/pybind11#5542 (merged on Mar 5, 2025)

If that's true, you will not have to do anything special in the future. For the moment, use pybind11 master floating. After the first pybind11 v3 release you can pin to releases if you wish.