net: remove internal support for obsolete DNS record types

net: remove internal support for obsolete DNS record types

There are no real world use cases for HINFO, MINFO, MB, MG, or MR
records, and package net's exposed APIs don't provide any way to
access them even if there were. If a use ever does show up, we can
revive them. In the mean time, this is just effectively-dead code that
sticks around because of rr_mk.

Change-Id: I6c188b5ee32f3b3a04588b79a0ee9c2e3e725ccc
Reviewed-on: https://go-review.googlesource.com/22593
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>