ReplicatedPG::cancel_copy: clear cop->obc

Otherwise, an objecter callback might still be hanging
onto this reference until after the flush.

Fixes: #8894
Introduced: 589b639af7c8834a1e6293d58d77a9c440107bc3
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 5040413054e923d6d5a2b4928162dba140d980e0)