Remove deprecated murmur3 checks

Description

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.

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Reporter

James Falcon

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