cmd/go: remove obsolete comment referring to deleted parameter

The dir parameter was removed in https://golang.org/cl/5732045.

Fixes #15503.

Change-Id: I02a6d8317233bea08633715a095ea2514822032b
Reviewed-on: https://go-review.googlesource.com/24011
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/build.go

@@ -2182,7 +2182,6 @@ func mkAbs(dir, f string) string {
 type toolchain interface {
 	// gc runs the compiler in a specific directory on a set of files
 	// and returns the name of the generated output file.
-	// The compiler runs in the directory dir.
 	gc(b *builder, p *Package, archive, obj string, asmhdr bool, importArgs []string, gofiles []string) (ofile string, out []byte, err error)
 	// cc runs the toolchain's C compiler in a directory on a C file
 	// to produce an output file.