cmd/gc: remove "send used as value" hint
cmd/gc: remove "send used as value" hint

This message was helpful for pre-Go 1 users updating to Go 1.
That time is past. Now the message is confusing because it
depends on knowing what pre-Go 1 looked like.

Update issue 4697.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13335051

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang