Don't copy extended statistics during MERGE/SPLIT partition operations

Don't copy extended statistics during MERGE/SPLIT partition operations

When MERGE/SPLIT created new partitions, it was cloning the extended
statistics of the parent table.

However, extended stats on partitioned tables don't behave like
indexes on partitioned tables (which exist only to create physical
indexes on child tables).  Rather, extended stats on a parent 1) cause
extended stats to be collected and computed across the whole partition
hierarchy, and 2) do not cause extended stats to be computed for the
individual partitions.

"CREATE TABLE ... PARTITION OF" command doesn't copy extended
statistics.  This commit makes createPartitionTable() behave
consistently.

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/ZiJW1g2nbQs9ekwK%40pryzbyj2023
Author: Alexander Korotkov, Justin Pryzby