drbd: Removed outdated comments and code that envisioned VNRs in header 95

Since have now header 100, that has space for 16 bit volume numbers,
the high byte of the length in header 95 is no longer reserved for
8 bit volume numbers.

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