wallet2: fix is_synced checking target height, not height

Target height would be appropriate for the daemon, which syncs
off other daemons, but the wallet syncs off the daemon it's
connected to, and its target is the daemon's current height.

src/wallet/wallet2.cpp

@@ -13212,7 +13212,7 @@ uint64_t wallet2::get_blockchain_height_by_date(uint16_t year, uint8_t month, ui
 bool wallet2::is_synced()
 {
   uint64_t height;
-  boost::optional<std::string> result = m_node_rpc_proxy.get_target_height(height);
+  boost::optional<std::string> result = m_node_rpc_proxy.get_height(height);
   if (result && *result != CORE_RPC_STATUS_OK)
     return false;
   return get_blockchain_current_height() >= height;