vclock: vclock_get - drop misleading masking

We rely heavily that VCLOCK_MAX is 32 bits wide
and using "VCLOCK_MAX - 1" as a mask for safe
access to the replica id is simply misleading.

Instead use assert here because we might change
the number of supported replicas one day and they
do not have to be pow2 value.

And no need for this completely useless comment.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>