Remove unused variables in monero codebase

There are quite a few variables in the code that are no longer
(or perhaps never were) in use. These were discovered by enabling
compiler warnings for unused variables and cleaning them up.

In most cases where the unused variables were the result
of a function call the call was left but the variable
assignment removed, unless it was obvious that it was
a simple getter with no side effects.

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -1849,10 +1849,8 @@ skip:
   bool t_cryptonote_protocol_handler<t_core>::should_download_next_span(cryptonote_connection_context& context, bool standby)
   {
     std::vector<crypto::hash> hashes;
-    boost::uuids::uuid span_connection_id;
     boost::posix_time::ptime request_time;
     boost::uuids::uuid connection_id;
-    std::pair<uint64_t, uint64_t> span;
     bool filled;
 
     const uint64_t blockchain_height = m_core.get_current_blockchain_height();
@@ -1878,7 +1876,6 @@ skip:
 
         // in standby, be ready to double download early since we're idling anyway
         // let the fastest peer trigger first
-        long threshold;
         const double dl_speed = context.m_max_speed_down;
         if (standby && dt >= REQUEST_NEXT_SCHEDULED_SPAN_THRESHOLD_STANDBY && dl_speed > 0)
         {