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_core/tx_pool.cpp

@@ -383,7 +383,6 @@ namespace cryptonote
   bool tx_memory_pool::add_tx(transaction &tx, tx_verification_context& tvc, relay_method tx_relay, bool relayed, uint8_t version)
   {
     crypto::hash h = null_hash;
-    size_t blob_size = 0;
     cryptonote::blobdata bl;
     t_serializable_object_to_blob(tx, bl);
     if (bl.size() == 0 || !get_transaction_hash(tx, h))
@@ -1041,7 +1040,6 @@ namespace cryptonote
       return true;
     }, true, category);
 
-    txpool_tx_meta_t meta;
     for (const key_images_container::value_type& kee : m_spent_key_images) {
       const crypto::key_image& k_image = kee.first;
       const std::unordered_set<crypto::hash>& kei_image_set = kee.second;