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

contrib/postgres_fdw/connection.c

@@ -271,7 +271,6 @@ GetConnection(UserMapping *user, bool will_prep_stmt, PgFdwConnState **state)
 			 entry->conn);
 		disconnect_pg_server(entry);
 
-		if (entry->conn == NULL)
 		make_new_connection(entry, user);
 
 		begin_remote_xact(entry);