Skip to main in tree view

## Description

The `skipToMain` link has been added in https://github.com/jupyter/notebook/pull/6844 to allow user to tab only once to reach the first Notebook cell.

But the link is also present in the tree view, and doesn't seem to do anything.

Should the link do something or should it be removed from the tree view ?

## Reproduce

Open the tree view and tab (once there is no click anywhere else).

## Expected behavior

The link to not exist or to redirect somewhere (maybe new notebook button, or the first notebook file in file browser...)

Reproduce the issue by opening the tree view and tabbing once, then inspect where the skipToMain link is rendered. Confirm whether it is intended to be removed or redirected to a useful tree-view destination, and verify that the resulting keyboard behavior matches the chosen outcome.

Written by the indexing model from the issue text.