[DomTree] Remove unnecessary domtree level check in SemiNCA (NFC) (#73107)

runSemiNCA() currently checks that the ReverseChildren are below
MinLevel in the DT, which is used when performing incremental updates.

However, ReverseChildren is populated during runDFS with only the
predecessors that are part of that DFS walk, which will itself be level
limited in the relevant cases. As such, I don't believe that this should
be checked during runSemiNCA().

This code probably dates back to a time when predecessors were not
cached during runDFS and as such not limited to the visited subtree
only.