Revert "Make pg_dump emit ATTACH PARTITION instead of PARTITION OF"

... and fallout (from branches 10, 11 and master).  The change was
ill-considered, and it broke a few normal use cases; since we don't have
time to fix it, we'll try again after this week's minor releases.

Reported-by: Rushabh Lathia
Discussion: https://postgr.es/m/CAGPqQf0iQV=PPOv2Btog9J9AwOQp6HmuVd6SbGTR_v3Zp2XT1w@mail.gmail.com