doc: remove mention of Windows junction points by pg_upgrade
pg_upgrade never used Windows junction points but instead always used
Windows hard links.
Reported-by: Adrian Klaver
Discussion: https://postgr.es/m/
6a638c60-90bb-4921-8ee4-
5fdad68f8b09@aklaver.com
Backpatch-through: 9.3, where the mention first appeared