pg_visibility: Use visibilitymap_count instead of loop.

pg_visibility: Use visibilitymap_count instead of loop.

This commit updates pg_visibility_map_summary() to use the
visibilitymap_count() API, replacing its own counting mechanism. This
simplifies the function and improves performance by leveraging the
vectorized implementation introduced in commit 41c51f0c68.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAEze2WgPu-EYYuYQimy=AHQHGa7w8EvLVve5DM5eGMR6zh-7sw@mail.gmail.com