cryptohome: remove last reference to hidden_mount

The actual usage of the field is removed as of
https://chromium-review.googlesource.com/c/chromium/src/+/2513592

And after this CL the field is deprecated in
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2594928

Bug: none
Test: none
Change-Id: If86b0b65c9dd28b25d68778d640380dd55b46b21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596276Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Daniil Lunev <dlunev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840056}

chromeos/dbus/cryptohome/fake_cryptohome_client.h

@@ -246,7 +246,6 @@ class COMPONENT_EXPORT(CRYPTOHOME_CLIENT) FakeCryptohomeClient
   bool to_migrate_from_ecryptfs() const {
     return last_mount_request_.to_migrate_from_ecryptfs();
   }
-  bool hidden_mount() const { return last_mount_request_.hidden_mount(); }
   bool public_mount() const { return last_mount_request_.public_mount(); }
   const cryptohome::AuthorizationRequest& get_last_mount_authentication()
       const {