This sentence is partially guilty why httptrace is considered as an
http.Client tracing package. Removing the mention.

Updates #17152.

Change-Id: I69f78a6e10817db933f44e464a949ae896e44ec6
Reviewed-on: https://go-review.googlesource.com/29755Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>