Revert "MB 87: Disable the default browser box on Windows and Linux"

This reverts commit bd64d5f0.

We are going to disable the default browser whenever we are in portable
mode instead.

browser/components/preferences/main.js

@@ -1744,9 +1744,7 @@ var gMainPane = {
       let defaultBrowserBox = document.getElementById("defaultBrowserBox");
       let isInFlatpak = gGIOService?.isRunningUnderFlatpak;
       // Flatpak does not support setting nor detection of default browser
-      // privacy-browser#87: Let's keep the default browser box only on macOS
-      // for now.
-      if (!shellSvc || isInFlatpak || Services.appinfo.OS !== "Darwin") {
+      if (!shellSvc || isInFlatpak) {
         defaultBrowserBox.hidden = true;
         return;
       }