cmd/go/internal/load: remove redundant assignment to BinDir

This assignment became a no-op in CL 36196, where both it and the
preceding assignment were changed to cfg.GOROOTbin (from gorootBin and
gobin respectively).

Change-Id: If74969c4cc3ffc5d8394ff9d8e8bcec9e0a4e3b0
Reviewed-on: https://go-review.googlesource.com/c/151561


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