osd: Remove bogus assert(active == acting.size())

We saw this assert because active is not correctly computed.
Remove assert and incorrectly computed active count.
We already use acting.size() to determine whether to set PG_STATE_DEGRADED.

Fixes: #6896

Signed-off-by: David Zafman <david.zafman@inktank.com>
Merge pull request #868 from ceph/wip-6896

osd: Remove bogus assert(active == acting.size())

Reviewed-by: Samuel Just <sam.just@inktank.com>