cleanup(netx): remove subpackages and unnecessary code (#788)

This pull request consists of several small and obvious cleanups in the netx directory.

See https://github.com/ooni/probe/issues/2121

internal/engine/netx/resolver/cache.go

@@ -1,49 +0,0 @@
-package resolver
-
-import (
-	"context"
-	"sync"
-
-	"github.com/ooni/probe-cli/v3/internal/model"
-)
-
-// CacheResolver is a resolver that caches successful replies.
-type CacheResolver struct {
-	ReadOnly bool
-	model.Resolver
-	mu    sync.Mutex
-	cache map[string][]string
-}
-
-// LookupHost implements Resolver.LookupHost
-func (r *CacheResolver) LookupHost(
-	ctx context.Context, hostname string) ([]string, error) {
-	if entry := r.Get(hostname); entry != nil {
-		return entry, nil
-	}
-	entry, err := r.Resolver.LookupHost(ctx, hostname)
-	if err != nil {
-		return nil, err
-	}
-	if !r.ReadOnly {
-		r.Set(hostname, entry)
-	}
-	return entry, nil
-}
-
-// Get gets the currently configured entry for domain, or nil
-func (r *CacheResolver) Get(domain string) []string {
-	r.mu.Lock()
-	defer r.mu.Unlock()
-	return r.cache[domain]
-}
-
-// Set allows to pre-populate the cache
-func (r *CacheResolver) Set(domain string, addresses []string) {
-	r.mu.Lock()
-	if r.cache == nil {
-		r.cache = make(map[string][]string)
-	}
-	r.cache[domain] = addresses
-	r.mu.Unlock()
-}