pg_dump: Remove dead code in TAP tests

pg_dump: Remove dead code in TAP tests

The schema_only_with_statistics test scenario was referenced in
002_pg_dump.pl, but was associated to no command sequence since
0ed92cf50cc4.

Issue discovered while investigating a different bug.  Perhaps this
cleanup is not worth backpatching, but there is also an argument in
favor of reducing noise when touching this area of the code in stable
branches.

Reviewed-by: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/ai-y0S7Z25NlrG_n@paquier.xyz
Backpatch-through: 18