macosx: the window should not be always the main window

With this patch, the window has the proper inactive state when another window is in foreground

Close #6016
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>
(cherry picked from commit 6e78f7b4d206c2d76b2d09e953383f0150cd051f)

modules/gui/macosx/MainWindow.m

@@ -789,12 +789,6 @@ static VLCMainWindow *_o_sharedInstance = nil;
     return [super validateMenuItem:menuItem];
 }
 
-- (BOOL)isMainWindow
-{
-
-return YES;
-}
-
 - (void)setTitle:(NSString *)title
 {
     if (b_dark_interface)