Now that specific menus can be searched directly (see 7f9d51853c),
there is no need to maintain separate search functionality for adding
nodes. This PR removes the add node search. In a way this brings us
closer to the `NodeItem` situation before, but the setup is more
flexible since the menus are more standard and easier to customize.
In the few ways we customized the node search items before, this gives
us the same results as before. Overall the searching is less flexible,
but I think that is jsut a tradeoff we have to accept for the simplicity
of searching menus. In the future menus could be made more dynamic,
with each builtin node's menu path stored on the node type, similar to
assets. That might be a nice compromise. In the meantime this code
is just dead weight.
