pg_dump: Simplify query for retrieving attribute statistics.

pg_dump: Simplify query for retrieving attribute statistics.

This query fetches information from pg_stats, which did not return
table OIDs until recent commit 3b88e50d6c.  Because of this, we had
to cart around arrays of schema and table names, and we needed an
extra filter clause to hopefully convince the planner to use the
correct index.  With the introduction of pg_stats.tableid, we can
instead just use an array of OIDs, and we no longer need the extra
filter clause hack.

Author: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CADkLM%3DcoCVy92QkVUUTLdo5eO2bMDtwMrzRn_8miAhX%2BuPaqXg%40mail.gmail.com