cmd/go: remove MallocNanoZone environment variable workaround

We added a workaround in runtime/race. This should not be
necessary now.

Updates #49138.

Change-Id: Ia2010e4acc95c4ddf5f463ab2919401d893c0bac
Reviewed-on: https://go-review.googlesource.com/c/go/+/374314
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/script_test.go

@@ -146,7 +146,6 @@ var extraEnvKeys = []string{
 	"GO_TESTING_GOTOOLS", // for gccgo testing
 	"GCCGO",              // for gccgo testing
 	"GCCGOTOOLDIR",       // for gccgo testing
-	"MallocNanoZone",     // Needed to work around an apparent kernel bug in macOS 12; see https://golang.org/issue/49138.
 }
 
 // setup sets up the test execution temporary directory and environment.