util: remove libevent <= 2.0.18 back-compat code

Now that we require libevent >=2.0.21, remove backwards compatibility
code for older versions.

src/bitcoin-cli.cpp

@@ -93,9 +93,6 @@ static void SetupCliArgs(ArgsManager& argsman)
 /** libevent event log callback */
 static void libevent_log_cb(int severity, const char *msg)
 {
-#ifndef EVENT_LOG_ERR // EVENT_LOG_ERR was added in 2.0.19; but before then _EVENT_LOG_ERR existed.
-# define EVENT_LOG_ERR _EVENT_LOG_ERR
-#endif
     // Ignore everything other than errors
     if (severity >= EVENT_LOG_ERR) {
         throw std::runtime_error(strprintf("libevent error: %s", msg));

src/httpserver.cpp

@@ -338,10 +338,6 @@ static void HTTPWorkQueueRun(WorkQueue<HTTPClosure>* queue, int worker_num)
 /** libevent event log callback */
 static void libevent_log_cb(int severity, const char *msg)
 {
-#ifndef EVENT_LOG_WARN
-// EVENT_LOG_WARN was added in 2.0.19; but before then _EVENT_LOG_WARN existed.
-# define EVENT_LOG_WARN _EVENT_LOG_WARN
-#endif
     if (severity >= EVENT_LOG_WARN) // Log warn messages and higher without debug category
         LogPrintf("libevent: %s\n", msg);
     else