Revert "Accept any XDG_ environment variable to determine desktop (#2037)" (#2042)

This reverts commit 67e2185964.

client/cmd/login_test.go

@@ -2,7 +2,6 @@ package cmd
 
 import (
 	"fmt"
-	"runtime"
 	"strings"
 	"testing"
 
@@ -52,16 +51,3 @@ func TestLogin(t *testing.T) {
 		t.Errorf("expected non empty Private key, got empty")
 	}
 }
-
-func TestIsLinuxRunningDesktop(t *testing.T) {
-	if runtime.GOOS != "linux" {
-		t.Skip("skipping test on non-linux platform")
-	}
-
-	t.Setenv("XDG_FOO", "BAR")
-
-	isDesktop := isLinuxRunningDesktop()
-	if !isDesktop {
-		t.Errorf("expected desktop environment, got false")
-	}
-}