ReplicatedPG: Removed the redundant register_snapset_context call

In the get_object_context(), the get_snapset_context is been called
and the register_snapset_context is already been invoked from there.
This call seems to be redundant.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
Merge pull request #2062 from somnathr/wip-sd-replicated-pg

ReplicatedPG: Removed the redundant register_snapset_context call

Reviewed-by: Samuel Just <sam.just@inktank.com>