all: remove unnecessary ", _" from map reads

all: remove unnecessary ", _" from map reads

If the bool value isn't used, there is no need to assign to underscore -
there is a shorter form that only returns the value and behaves in the
exact same way.

Change-Id: Iaf801b8e966da6c2f565bc39e3bb028175c92d60
Reviewed-on: https://go-review.googlesource.com/40920
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>