doc: Remove notes about "unencrypted" passwords.
The documentation for the pg_authid system catalog and the
pg_shadow system view indicates that passwords might be stored in
cleartext, but that hasn't been possible for some time.
Oversight in commit
eb61136dc7.
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aD2yKkZro4nbl5ol%40nathan
Backpatch-through: 13