get_blockchain_top now returns void

It was always returning true, and could not be foreseen to
usefully return errors in the future. This silences CID 162652
as well as saves some checking code in a few places.

tests/core_proxy/core_proxy.cpp

@@ -229,10 +229,9 @@ bool tests::proxy_core::get_short_chain_history(std::list<crypto::hash>& ids) {
     return true;
 }
 
-bool tests::proxy_core::get_blockchain_top(uint64_t& height, crypto::hash& top_id) {
+void tests::proxy_core::get_blockchain_top(uint64_t& height, crypto::hash& top_id) {
     height = 0;
     top_id = get_block_hash(m_genesis);
-    return true;
 }
 
 bool tests::proxy_core::init(const boost::program_options::variables_map& /*vm*/) {