cmd/go: fix go list usage line

CL 108156 added -cgo and -export,
but in the usage line it added -cgo and -list.
CL 117015 correctly added -export to the usage line.
All that remains is to remove -list.

Change-Id: I8cc5cfc78bc6b52080ae1b861f92620a8f18b53f
Reviewed-on: https://go-review.googlesource.com/118375


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>