Signal handling refactor

This is a removal of legacy code for signal handling. This signal
handling was useful for the legacy runner where all tasks were running
in child processes. But now when we use spawners for creating tasks each
with different implementation, we need to handle the signal handling in
the spawners itself.

Signed-off-by: Jan Richter <jarichte@redhat.com>

Author: richtja

avocado/core/app.py

@@ -25,7 +25,6 @@
 from avocado.core.output import STD_OUTPUT
 from avocado.core.parser import Parser
 from avocado.core.settings import settings
-from avocado.utils import process
 
 
 class AvocadoApp:
@@ -90,13 +89,6 @@ def _run_cli_plugins(self):
 
     @staticmethod
     def _setup_signals():
-        def sigterm_handler(signum, frame):  # pylint: disable=W0613
-            children = process.get_children_pids(os.getpid())
-            for child in children:
-                process.kill_process_tree(int(child))
-            raise SystemExit("Terminated")
-
-        signal.signal(signal.SIGTERM, sigterm_handler)
         if hasattr(signal, "SIGTSTP"):
             signal.signal(signal.SIGTSTP, signal.SIG_IGN)  # ignore ctrl+z