cmd/dist: revert mod=vendor change accidentally included in CL 164623

I thought I had removed all of the 'mod=vendor' hacks I had inserted
previously, but apparently missed this one in a bad merge or rebase.

Updates #30228

Change-Id: Ia5aea754bf986458373fefd08fa9dd3941e31e43
Reviewed-on: https://go-review.googlesource.com/c/go/+/167077
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/dist/build.go

@@ -192,21 +192,6 @@ func xinit() {
 
 	gogcflags = os.Getenv("BOOT_GO_GCFLAGS")
 
-	// Add -mod=vendor to GOFLAGS so that commands won't try to resolve go.mod
-	// files for vendored external modules.
-	// TODO(golang.org/issue/30240): If the vendor directory contains the go.mod
-	// files, this probably won't be necessary.
-	// TODO(golang.org/issue/26849): Escape spaces in GOFLAGS if needed.
-	goflags := strings.Fields(os.Getenv("GOFLAGS"))
-	for i, flag := range goflags {
-		if strings.HasPrefix(flag, "-mod=") {
-			goflags = append(goflags[0:i], goflags[i+1:]...)
-			break
-		}
-	}
-	goflags = append(goflags, "-mod=vendor")
-	os.Setenv("GOFLAGS", strings.Join(goflags, " "))
-
 	cc, cxx := "gcc", "g++"
 	if defaultclang {
 		cc, cxx = "clang", "clang++"