migration: Remove qmp_migrate_finish

migration: Remove qmp_migrate_finish

After cleanups, the qmp_migrate_finish function is now just a call to
migration_connect_outgoing(). Remove qmp_migrate_finish() and rename
the qmp_migrate_finish_cb callback.

This also allows the function's error handling to be removed as it now
receives &local_err like the rest of the callees of qmp_migrate().

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Link: https://lore.kernel.org/qemu-devel/20260123141656.6765-25-farosas@suse.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>