net: remove TODO for DNS cache

The proposal to add a DNS cache was rejected, so there is no longer a
need for the associated TODO.

Updates #24796

Change-Id: Ifcedcff72c75a70b2143de0bd3f7bf85ac3528f6
Reviewed-on: https://go-review.googlesource.com/c/145197
Run-TryBot: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.public.networking@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix.go

@@ -9,7 +9,6 @@
 
 // TODO(rsc):
 //	Could potentially handle many outstanding lookups faster.
-//	Could have a small cache.
 //	Random UDP source port (net.Dial should do that for us).
 //	Random request IDs.