postgres_fdw: Remove useless if-test in GetConnection().

postgres_fdw: Remove useless if-test in GetConnection().

Checking whether entry->conn is NULL after doing disconnect_pg_server()
for that entry is pointless, as that function ensures that it is NULL.

Thinko in commit 7fc1a81e4; this would be harmless, so patch HEAD only.

Reviewed-by: Richard Guo and Daniel Gustafsson
Discussion: https://postgr.es/m/CAPmGK169vQ83PQwQkoxO-AK2EeK1EsgsxixedM%2BBLWEAhZ_AqQ%40mail.gmail.com