Fix `edges` function for rooted trees to return one sequence of edges

It was returning sequence of sequences of edges.
Fix the helper remove-vertices function for rooted trees

The acc in the reduce should always be a tree. The problem was that it
was becoming nil when the if condition is false.
Add docstring to the helper remove-vertices function
Make remove-vertices dissoc the vertex from the :children field

If this is not done, the vertices function will still count it in.