Remove unneeded extra stack trace variable

This was intended for a new option in the top-level Exception handler dialog, but extraneous here.

megamek/src/megamek/MegaMek.java

@@ -62,7 +62,6 @@ public static void main(String... args) {
         Thread.setDefaultUncaughtExceptionHandler((thread, t) -> {
             LogManager.getLogger().error("Uncaught Exception Detected", t);
             final String name = t.getClass().getName();
-            final StackTraceElement[] stack = t.getStackTrace();
             JOptionPane.showMessageDialog(null,
                     String.format("Uncaught %s detected. Please open up an issue containing all logs, the game save file, and customs at https://github.com/MegaMek/megamek/issues", name),
                     "Uncaught " + name, JOptionPane.ERROR_MESSAGE);
@@ -357,4 +356,4 @@ public static void initializeSuiteGraphicalSetups(final String currentProject) {
         // Setup Button Order Preferences
         ButtonOrderPreferences.getInstance().setButtonPriorities();
     }
-}
+}