all: remove ipv6 check polling

netmon provides ipv6 availability during network event changes, so use
this metadata instead of wasting on polling check.

Further, repeated network errors will force marking ipv6 as disable if
were being enabled, catching a rare case when ipv6 were disabled from
cli or system settings.

cmd/cli/dns_proxy.go

@@ -556,6 +556,10 @@ func (p *prog) proxy(ctx context.Context, req *proxyRequest) *proxyResponse {
 			if errors.As(err, &e) && e.Timeout() {
 				upstreamConfig.ReBootstrap()
 			}
+			// For network error, turn ipv6 off if enabled.
+			if ctrld.HasIPv6() && (errUrlNetworkError(err) || errNetworkError(err)) {
+				ctrld.DisableIPv6()
+			}
 		}
 
 		return nil