[ios] Remove unused constant

Bug: none
Change-Id: I07c797cab7f97428ea17ec30fe33aa9739553d99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632945
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844753}

ios/chrome/browser/sessions/session_service_ios.h

@@ -13,10 +13,6 @@
 @class SessionIOS;
 @class SessionIOSFactory;
 
-namespace session_constants {
-NSString* const kSessionsDirectory = @"Sessions";
-}
-
 // A singleton service for saving the current session. Can either save on a
 // delay or immediately. Saving is always performed on a separate thread.
 @interface SessionServiceIOS : NSObject