Remove HTTP trigger `base` option

[lann]

The app-wide HTTP trigger base option effectively prepends a fixed string to every HTTP trigger route. This is almost identical to the equivalent routes, but a couple of the magic spin-* headers expose this information to the guest as well.

A search of Github suggests that this option is practically unused after it was made optional in #1803. I'd like to remove this option with Spin 3.0 and redefine or remove related magic spin-* headers.