Dead reference to schema_only_with_statistics in pg_dump TAP code

Hi all,

While looking at the TAP tests of pg_dump today, I have bumped into a
dead reference to a command sequence named schema_only_with_statistics.
This reference has been removed in 0ed92cf50cc4, and has been listed a
couple of times after that, having no effect.

Let's clean up that as of the attached on HEAD, at least.  v18 would
be nice to include in the set, as this script is already complicated
and that could create noise when backpatching.

Comments or opinions?
--
Michael