fix: remove static from type_has_shared_from_this

[lucascolley]

I hit the following warning when trying to build SciPy with clang-23 in scipy/scipy#26041:

In file included from /home/runner/work/scipy/scipy/.pixi/envs/build-clang-23/include/pybind11/trampoline_self_life_support.h:8:
In file included from /home/runner/work/scipy/scipy/.pixi/envs/build-clang-23/include/pybind11/detail/using_smart_holder.h:8:
/home/runner/work/scipy/scipy/.pixi/envs/build-clang-23/include/pybind11/detail/struct_smart_holder.h:71:13: error: unused function template 'type_has_shared_from_this' [-Werror,-Wunused-template]
   71 | static auto type_has_shared_from_this(const T *ptr)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~

Claude code says that these static qualifiers are incorrect, since including them on a function at namespace scope implies that they are only visible in this translation unit.

[rwgk]

Thanks for reporting this issue. As currently submitted, this PR does not make the motivating Clang 23 build pass on its own: it does not include the changes from #6155, so including pybind11/pybind11.h still fails with -Werror,-Wunused-template at type_has_shared_from_this.

Could you please consolidate all changes required for the reported SciPy build into this PR, including any additional warnings encountered, and rerun the original build to confirm that it completes successfully?

Could you please also provide the exact compiler details — ideally the complete clang++ --version output, maybe also the package/channel or LLVM commit used, and the relevant build command and compiler flags? Since Clang 23 is currently a pre-release compiler, that information would make the issue reproducible and help confirm that the fix is complete. Thanks!

[lucascolley]

I replied at #6156 (comment)

[rwgk]

Closing in favor of #6156.