ACCUMULO-3342 Remove check for tracer in ZK

  Removes the MiniAccumuloCluster check to see if an existing instance is
  running via the existance of a tracer registration. Tracers aren't really part
  of "Accumulo", so much as an extra service, which also registers itself in ZK.
  As such, they don't really determine whether Accumulo is "running", and in any
  case, it is not running by default in MiniAccumuloCluster for the tests that
  are failing, so it's expected that this directory will not exist.