Removed RollbackOnlyIfRestorePossible option from DeviceRollbackToTargetVersion policy

As this option was not implemented and won't be launched, so it should be removed from documentation and codebase before feature launch.

BUG=chromium:947621
TEST=./build_packages --board=amd64-generic && \
cros_run_unit_tests --board=amd64-generic --packages update_engine

Change-Id: I11ffeb7b65f55e9b026bd4673d16900c34627b34
Reviewed-on: https://chromium-review.googlesource.com/1619795
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Aya Elsayed <ayaelattar@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/update_manager/rollback_prefs.h b/update_manager/rollback_prefs.h
index 11d09d6..9567701 100644
--- a/update_manager/rollback_prefs.h
+++ b/update_manager/rollback_prefs.h
@@ -31,9 +31,8 @@
   kDisabled = 1,
   kRollbackAndPowerwash = 2,
   kRollbackAndRestoreIfPossible = 3,
-  kRollbackOnlyIfRestorePossible = 4,
   // This value must be the last entry.
-  kMaxValue = 5
+  kMaxValue = 4
 };
 
 }  // namespace chromeos_update_manager