MINOR: Do not wait for first line of console consumer output since we now have a more reliable test using JMX

Waiting for the first line of output was added in KAFKA-2527 when JmxMixin was originally added as a heuristic to
determine when the process was ready. We've since determined this is not good enough given JmxTool's limitations
and now include a separate, more reliable check before starting JmxTool. This check is also dangerous since a
consumer that is started before data is available in the topic, it won't output anything to stdout and only logs
errors to a separate log file. This means we may have a long delay between starting the process and starting JMX
monitoring.

Since we have a more reliable check for liveness via JMX now (and in cases that need it, partition assignment
metrics via JMX), we should no longer need to wait for the first line of output.

Author: Ewen Cheslack-Postava <me@ewencp.org>

Reviewers: Ismael Juma <ismael@juma.me.uk>, Apurva Mehta <apurva@confluent.io>

Closes #3447 from ewencp/dont-wait-first-line-console-consumer

tests/kafkatest/services/console_consumer.py

@@ -255,14 +255,12 @@ class ConsoleConsumer(KafkaPathResolverMixin, JmxMixin, BackgroundThreadService)
         self.logger.debug("Console consumer %d command: %s", idx, cmd)
 
         consumer_output = node.account.ssh_capture(cmd, allow_fail=False)
-        first_line = next(consumer_output, None)
 
-        if first_line is not None:
-            self.logger.debug("collecting following jmx objects: %s", self.jmx_object_names)
         self.init_jmx_attributes()
+        self.logger.debug("collecting following jmx objects: %s", self.jmx_object_names)
         self.start_jmx_tool(idx, node)
 
-            for line in itertools.chain([first_line], consumer_output):
+        for line in consumer_output:
             msg = line.strip()
             if msg == "shutdown_complete":
                 # Note that we can only rely on shutdown_complete message if running 0.10.0 or greater