cmd/go/internal/envcmd: remove duplicate env value set

CL 586095 wasn't needed because setting the GODEBUG value was just moved
lower in CL 563137. Now we set it twice. Remove the second set so that
it's only set once.

Change-Id: I4a830977833b007e996d0ccaec705a1da4631e6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/586477
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/envcmd/env.go

@@ -132,7 +132,6 @@ func MkEnv() []cfg.EnvVar {
 				env[i].Changed = true
 			}
 		case "GODEBUG":
-			env[i].Value = os.Getenv("GODEBUG")
 			env[i].Changed = env[i].Value != ""
 		}
 	}