libweave: Remove unused local constant

This causes on clang.

Change-Id: I8bdffc52e9000c4033927555032b8b8294b80cfd
Reviewed-on: https://weave-review.googlesource.com/3172
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
(cherry picked from commit daeafc6e42c1acb36aae290e60462e47947fd643)
Reviewed-on: https://weave-review.googlesource.com/3173
diff --git a/src/device_registration_info.cc b/src/device_registration_info.cc
index 195895f..b7ff589 100644
--- a/src/device_registration_info.cc
+++ b/src/device_registration_info.cc
@@ -39,7 +39,6 @@
 namespace {
 
 const int kPollingPeriodSeconds = 7;
-const int kBackupPollingPeriodMinutes = 30;
 
 namespace fetch_reason {