Remove the fetch route, handler & gomod parser

[arschles]

The fetch route is here and the handler is here

This handler was used by the CLI (which is obsolete and was removed), and we don't need it anymore. This issue is for removing the fetch handler, the fetch route, and the gomod parser which was used by the fetch handler

[ryancox]

is there a gomod parser library elsewhere that could be used / improved?

[arschles]

@ryancox yes, it's in a package inside golang/go, I think an internal package. We're going to delegate as much of this as possible to calling the go CLI so we don't have to depend on code inside of the go toolchain

cc/ @rsc @marwan-at-work, both of whom have been part of discussions on this topic

[arschles]

This is a subset of #335, closing