Don't export bs_thread_pool (#16398)

## Description
cudf does not currently export any headers that depend on
bs_thread_pool, and having it as a dependency is currently causing
problems for consumers. Avoid exporting it since it's not needed.

## Checklist
- [ ] I am familiar with the [Contributing
Guidelines](https://github.com/rapidsai/cudf/blob/HEAD/CONTRIBUTING.md).
- [ ] New or existing tests cover these changes.
- [ ] The documentation is up to date with these changes.

cpp/cmake/thirdparty/get_thread_pool.cmake

@@ -18,7 +18,7 @@ function(find_and_configure_thread_pool)
   include(${rapids-cmake-dir}/cpm/bs_thread_pool.cmake)
 
   # Find or install thread-pool
-  rapids_cpm_bs_thread_pool(BUILD_EXPORT_SET cudf-exports INSTALL_EXPORT_SET cudf-exports)
+  rapids_cpm_bs_thread_pool()
 
 endfunction()