Wagi modules ignore the `_routes` function

[radu-matei]

Currently, if a Wagi module exports a _routes function to declare sub-routes in the module (ref https://github.com/deislabs/wagi/blob/main/docs/writing_modules.md#advanced-declaring-sub-routes-in-the-module), that is ignored by Spin.

[radu-matei]

We have decided not to implement the _routes functionality from Wagi, as it significantly complicated routing, and it was not a widely used feature.