Branch Creator Tidying

[danieldresser-ie]

When initially working on #5984, I had hoped that it could operate similarly to BranchCreator ( which turned out not to be true ).

While looking through the code in detail, these two changes felt clearer to me:

• closestExistingPath returned a path, but the strings in that path were never used. existingPathLength just returns the length, which is the only thing we used.
• We stored depth in the tree of Locations in BranchesData, but this depth is just the number of nodes we visit during a traversal - there's no need to store an extra value.