ReplicatedPG: do not pass cop into C_Copyfrom

We do not know when the objecter will finally let go of this Context.  Thus, we
cannot know whether it will happen before the flush, at which point the
object_context held by the cop must have been released.

Also, we simply don't need it, process_copy_chunk alrady works in terms of the
tid!

Fixes: #8894
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Samuel Just <sam.just@inktank.com>