Remove unused skip_double_compressed_messages

[jeffwidman]

This skip_double_compressed_messages flag was added in #755 in
order to fix #718.

However, grep'ing through the code, it looks like it this is no longer
used anywhere and doesn't do anything.

So removing it.

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This change is 

[jeffwidman]

Note: This is a backwards-incompatible change.

If someone creates a consumer with this kwarg after we remove it, then KafkaConsumer will raise an exception:

kafka-python/kafka/consumer/group.py

Lines 309 to 312 in 70ea4c1

	# Only check for extra config keys in top-level class
	extra_configs = set(configs).difference(self.DEFAULT_CONFIG)
	if extra_configs:
	raise KafkaConfigurationError("Unrecognized configs: %s" % (extra_configs,))

The simplest solution is just wait to merge this until the 2.0 release.

[dpkp]

I think it is fine to land now. If you want to leave in a default value (undocumented / unused) to avoid the configuration error, that's reasonable. The supporting code for this config was removed in #1252 / commit fbea5f04bccd28f3aa15a1711548b131504591ac, so the config is and has been a no-op for a while.

[jeffwidman]

Sounds good, I'd rather land it now.