[iOS] Remove unused command

This CL removes the -closeAllIncognitoTabs command as it is not used.

Bug: 902781
Change-Id: I583e53095fcddf03c899d6b44779b38daced32a3
Reviewed-on: https://chromium-review.googlesource.com/c/1323670Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606387}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -4570,11 +4570,6 @@ applicationCommandEndpoint:(id<ApplicationCommands>)applicationCommandEndpoint {
   _voiceSearchController->PrepareToAppear();
 }
 
-- (void)closeAllIncognitoTabs {
-  DCHECK(self.isOffTheRecord);
-  [self.tabModel closeAllTabs];
-}
-
 #if !defined(NDEBUG)
 - (void)viewSource {
   Tab* tab = [_model currentTab];

ios/chrome/browser/ui/commands/browser_commands.h

@@ -61,9 +61,6 @@ class GURL;
 // Closes all tabs.
 - (void)closeAllTabs;
 
-// Closes all incognito tabs.
-- (void)closeAllIncognitoTabs;
-
 #if !defined(NDEBUG)
 // Shows the source of the current page.
 - (void)viewSource;