HPCC-12625 Fix regression in last commit for this issue.

Sub-subscriptions broken by last commit

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
How many elements are normally added to subs, and what proportion tend to be retained?  If a large proportion are retained then removing one item at a time might be O(N^2).  If this function is a hotspot for the profiling then it might be worth improving it.