Remove extra go.mod files

Only the top level build/soong/go.mod file should exist, any extras
breaks `go test ./...` in build/soong, and also breaks the kythe build
that produces xrefs for code search.

Bug: 315537199
Test: go test ./...

Ignore-AOSP-First: CPing test_spec rule to udc-mainline-prod to support migration of test targets. Cherry pick of:aosp/2847033

Change-Id: Id5262550654ff1559cfb3fa1ecdae95c126109df
Merged-In: Id5262550654ff1559cfb3fa1ecdae95c126109df