cmd/dist: cleanup message about building go_bootstrap

At this stage, dist is only building go_bootstrap as cmd/compile and
the rest of the Go toolchain has already been built.

Change-Id: I6f99fa00ff1d3585e215f4ce84d49344c4fcb8a5
Reviewed-on: https://go-review.googlesource.com/12779Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/build.go

@@ -1053,7 +1053,7 @@ func cmdbootstrap() {
 		xprintf("\n")
 	}
 
-	xprintf("##### Building compilers and go_bootstrap for host, %s/%s.\n", gohostos, gohostarch)
+	xprintf("##### Building go_bootstrap for host, %s/%s.\n", gohostos, gohostarch)
 	for _, dir := range buildorder {
 		install(dir)
 	}