Remove redundant $db->Insert_ID call

We use the mysqli driver for transactional support and I run into small issues with Concrete5 every now and then.  Here is the latest:

$newCID is retrieved in the original insert (line 505) and doesn't need to be overwritten.  Calling $db->Insert_ID() with the mysqli driver on that type of query (line 514) returns 0, which causes a problem with PageStatistics::incrementParents and movePageDispayOrdertoBottom().

I've submitted a couple of these changes before under NazWeb@Concrete5.org.