drbd: Magic reserved block_id value cleanup

The ID_VACANT definition has become entirely irrelevant by now.

The is_syncer_block_id() macro does not improve the code, so eliminated
it.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>