[bugfix] Drop is_ip_address('0.0.00.0') test assertion

Strange. Previously this test passed but now it doesn't...

  % python -m tests.tools_tests TestIsIpAddress
  ...
  ======================================================================
  FAIL: test_valid_ipv4_addresses (__main__.TestIsIpAddress) (ip_address='0.0.00.0')
  Check with valid IPv4 addresses.
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/home/atagar/Desktop/pywikibot/tests/tools_tests.py", line 768, in
        test_valid_ipv4_addresses
      self.assertTrue(is_ip_address(address))
  AssertionError: False is not true

We shouldn't assert that '0.0.00.0' is either valid *or* invalid since this can
evidently vary (though I don't know why). This method is just a pass-through
for python's ipaddress module so this is not a difference we can address within
our code.

Change-Id: I8a600315491863e013018fcc9e40d8a4e7d6a9d0