internal/secrets: remove test that depends on ambient permissions

It's inherently flaky.

Change-Id: I2f35d94a335f22943ffe8357a7f84a686b21d6e8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/257097
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>

internal/secrets/secrets_test.go

@@ -23,8 +23,4 @@ func TestGet(t *testing.T) {
 	if got != want {
 		t.Errorf("got %q, want %q", got, want)
 	}
-
-	if _, err := Get(context.Background(), "worker-database-password"); err == nil {
-		t.Error("was able to get a secret that we shouldn't")
-	}
 }