delete, do not clear, flushed timers

They are already reinitialized as needed, and deleting them leaves
the system primed for an IndexError:

  File ".../py-statsd/pystatsd/server.py", line 111, in process
      self.timers[key][0].append(float(fields[0] or 0))

...because there will be no [0] in the timer.  Fortunately, timer
lists are initialized as needed, so deleting this seems to make
everything do the right thing.