Revert "net: send EDNS(0) packet length in DNS query"

This reverts https://go.dev/cl/385035. For 1.18 we will use a simple
change to increase the accepted DNS packet size, to handle what appear
to be broken resolvers that don't honor the 512 byte limit. For 1.19
we will restore CL 385035 to make a proper EDNS request, so that it
has more testing time before it goes out in a release.

For #6464
For #21160
For #44135
For #51127
For #51153

Change-Id: Ie4a0eb85ca0a6a73bee5cd4cfc6b7d2a15ef259f
Reviewed-on: https://go-review.googlesource.com/c/go/+/386014


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Damien Neil <dneil@google.com>