Revert hardcoded check for track 6 when quitting to menu

This reverts commit 48313169b6, "Don't
fade music out when returning to the menu if it's Presenting VVVVVV".

This commit is being reverted because it is only a single-case patch -
that is, it fixes only a single symptom of the bug, and not its
underlying cause.

desktop_version/src/Input.cpp

@@ -2319,9 +2319,7 @@ void mapmenuactionpress()
         //This fixes an apparent frame flicker.
         FillRect(graphics.tempBuffer, 0x000000);
         graphics.fademode = 2;
-        if (music.currentsong != 6) {
-            music.fadeout();
-        }
+        music.fadeout();
         map.nexttowercolour();
         if (!game.glitchrunnermode)
         {