Remove preexec_fn hack from test cluster management

The hack is ugly, and the correct shutdown of the launched cluster
is alreadye handled by the atexit hook installed by the test harness.

asyncpg/cluster.py

@@ -36,21 +36,6 @@ def platform_exe(name):
         return name
 
 
-if _system == 'Linux':
-    def ensure_dead_with_parent():
-        import ctypes
-        import signal
-
-        try:
-            PR_SET_PDEATHSIG = 1
-            libc = ctypes.CDLL(ctypes.util.find_library('c'))
-            libc.prctl(PR_SET_PDEATHSIG, signal.SIGKILL)
-        except Exception as e:
-            print(e)
-else:
-    ensure_dead_with_parent = None
-
-
 def find_available_port(port_range=(49152, 65535), max_tries=1000):
     low, high = port_range
 
@@ -230,8 +215,7 @@ def start(self, wait=60, *, server_settings={}, **opts):
             self._daemon_process = \
                 subprocess.Popen(
                     [self._postgres, '-D', self._data_dir, *extra_args],
-                    stdout=stdout, stderr=subprocess.STDOUT,
-                    preexec_fn=ensure_dead_with_parent)
+                    stdout=stdout, stderr=subprocess.STDOUT)
 
             self._daemon_pid = self._daemon_process.pid