Fix non-native fullscreen quit to black screen bug

A previous change #1510 introduced a piece of debug code that
erroneously overrode the `window` property to always return the normal
window whereas in normal behavior it should return the full screen
window when in non-native full screen. This property was only added for
supporting unit tests and also completely unnecessary because the
superclass `NSWindowController` already provides it (the one that we
accidentally overrode). Just remove this.

Fix #1515

Author: ychin

src/MacVim/MMWindowController.h

@@ -58,7 +58,6 @@
 - (id)initWithVimController:(MMVimController *)controller;
 - (MMVimController *)vimController;
 - (MMVimView *)vimView;
-- (NSWindow *)window;
 - (NSString *)windowAutosaveKey;
 - (void)setWindowAutosaveKey:(NSString *)key;
 - (void)cleanup;

src/MacVim/MMWindowController.m

@@ -291,11 +291,6 @@ - (MMVimView *)vimView
     return vimView;
 }
 
-- (NSWindow *)window
-{
-    return decoratedWindow;
-}
-
 - (NSString *)windowAutosaveKey
 {
     return windowAutosaveKey;