Remove usage of datadog key as feature flag for enabling metrics

[hogaur]

Datadog key was earlier used as a feature flag to enable/disable metrics for ziggurat and the application using it. Earlier in a release, we introduced statsd key to do the same. This was done for brevity. This change denotes that Ziggurat supports stats metrics and not just metrics for Datadog. It also clears the confusion for the users who are not using Datadog but want to consume metrics by some other tool.

This issue is about removing the usage of the old datadog key from the code, config, and specs. Following places

• Remove fetching stats key in statsd-config function defined at https://github.com/gojek/ziggurat/blob/master/src/ziggurat/config.clj#L82
• Replace the datadog key with the statsd key in the default config map defined at https://github.com/gojek/ziggurat/blob/master/src/ziggurat/config.clj#L15
• Delete the test - "returns statsd config using the :datadog key" test written at https://github.com/gojek/ziggurat/blob/master/test/ziggurat/config_test.clj#L88
• Remove the mention of datadog key from the test - "returns statsd config using the :statsd key or :datadog key" written at https://github.com/gojek/ziggurat/blob/master/test/ziggurat/config_test.clj#L71
• Delete datadog test configuration file at https://github.com/gojek/ziggurat/blob/master/resources/config.test.datadog.only.edn