Revert "dashboard: test with -mmacosx-version-min set on darwin-amd64-10_15"

This reverts commit 015a2a0c58 (CL 382055).

Reason for revert: It's failing the cmd/dist staleness check very early in the run.

Change-Id: I6a0c674caaade46e20de15ba6a906a06eeb9cdf9
Reviewed-on: https://go-review.googlesource.com/c/build/+/382936
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Trust: Carlos Amedee <carlos@golang.org>
Run-TryBot: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Alex Rakoczy <alex@golang.org>
Run-TryBot: Alex Rakoczy <alex@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

dashboard/builders.go

@@ -2307,13 +2307,6 @@ func init() {
 		HostType:       "host-darwin-10_15",
 		distTestAdjust: macTestPolicy,
 		buildsRepo:     defaultPlusExpBuild,
-		env: []string{
-			// Find out via at least one builder if having -mmacosx-version-min
-			// set via CGO_CFLAGS causes unexpected problems. This env var will
-			// also be set by cmd/release during a release. See issue 50892.
-			"CGO_CFLAGS=-mmacosx-version-min=10.13",
-		},
-		Notes: "Unlike other darwin/amd64 builders, this one sets CGO_CFLAGS=-mmacosx-version-min=10.13 in env.",
 	})
 	addBuilder(BuildConfig{
 		Name:           "darwin-amd64-11_0",