net: use io.Discard in TestSendfileOnWriteTimeoutExceeded

net: use io.Discard in TestSendfileOnWriteTimeoutExceeded

Replace ioutil.Discard which is deprecated as of Go 1.16.

This was already done in CL 263142 but accidentially re-introduced in
CL 285914.

Change-Id: Ife0944d416294b1ba7c8f6b602aa68a3b9213c50
Reviewed-on: https://go-review.googlesource.com/c/go/+/299989
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>