DNS packing and unpacking uses hand-coded struct walking functions
rather than reflection, so these tags are unneeded and just contribute
to their runtime reflect metadata size.

Change-Id: I2db09d5159912bcbc3b482cbf23a50fa8fa807fa
Reviewed-on: https://go-review.googlesource.com/22594
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>