Remove incorrect assertion

It's legal for a client to not supply a specific version, e.g. not call
Cluster.Builder::withProtocolVersion, in which case
factory.protocolVersion is null. While v5 was marked beta this wasn't a
problem as Cluster.Builder::allowProtocolVersion was required, but now
this causes an NPE in the driver.

Author: adutra

driver-core/src/main/java/com/datastax/driver/core/Connection.java

@@ -1773,7 +1773,6 @@ private Message.ProtocolEncoder messageEncoderFor(ProtocolVersion version) {
    * v5.
    */
   void switchToV5Framing() {
-    assert factory.protocolVersion.compareTo(ProtocolVersion.V5) >= 0;
     // We want to do this on the event loop, to make sure it doesn't race with incoming requests
     assert channel.eventLoop().inEventLoop();