Remove deprecated murmur3 checks

We have old code around checking for murmur3. E.g.,
https://github.com/datastax/python-driver/blob/master/cassandra/cluster.py#L208
https://github.com/datastax/python-driver/blob/master/cassandra/metadata.py#L1619

These are just two examples, but there's more. We have a pure python fallback for our murmur3 implementation, so these and other murmur3 checks can be removed.