Remove useless mutex from BackendInterface and its usage from CassandraBackend (#326)

Fixes #304

src/backend/BackendInterface.h

@@ -89,9 +89,6 @@ protected:
     std::optional<LedgerRange> range;
     SimpleCache cache_;
 
-    // mutex used for open() and close()
-    mutable std::mutex mutex_;
-
 public:
     BackendInterface(boost::json::object const& config)
     {

src/backend/CassandraBackend.cpp

@@ -1001,7 +1001,6 @@ CassandraBackend::open(bool readOnly)
 
     BOOST_LOG_TRIVIAL(info) << "Opening Cassandra Backend";
 
-    std::lock_guard<std::mutex> lock(mutex_);
     CassCluster* cluster = cass_cluster_new();
     if (!cluster)
         throw std::runtime_error("nodestore:: Failed to create CassCluster");