Don't reset RTT on error in RTT monitor

This avoids ever having a 0 value for RTT

JAVA-3626

Author: jyemin

driver-core/src/main/com/mongodb/internal/connection/DefaultServerMonitor.java

@@ -398,7 +398,6 @@ public void run() {
                             connection.close();
                             connection = null;
                         }
-                        averageRoundTripTime.reset();
                     }
                     waitForNext();
                 }