libweave: Fix build break on Brillo

The private member |currently_online_| is no longer used and causes
the following error on Android/Brillo builds:

external/libweave/src/privet/wifi_bootstrap_manager.h:109:8:
error: private field 'currently_online_' is not used [-Werror,-Wunused-private-field]

Remove the field to fix the compile break.

Change-Id: I0830a4d12f777956638a12514565bb2270680e34
Reviewed-on: https://weave-review.googlesource.com/3220
Reviewed-by: Robert Ginda <rginda@google.com>

src/privet/wifi_bootstrap_manager.h

diff --git a/src/privet/wifi_bootstrap_manager.h b/src/privet/wifi_bootstrap_manager.h
index 62a77c2..468473f 100644
--- a/src/privet/wifi_bootstrap_manager.h
+++ b/src/privet/wifi_bootstrap_manager.h
@@ -105,8 +105,6 @@
   provider::Wifi* wifi_{nullptr};
   WifiSsidGenerator ssid_generator_;
   base::Time monitor_until_;
-
-  bool currently_online_{false};
   std::string privet_ssid_;
 
   // Helps to reset irrelevant tasks switching state.