Fix some more warnings from flake8

tests/rules/test_ssh_known_host.py

@@ -1,6 +1,5 @@
 import os
 import pytest
-from mock import Mock
 from thefuck.rules.ssh_known_hosts import match, get_new_command,\
     side_effect
 from tests.utils import Command

tests/rules/test_systemctl.py

@@ -1,4 +1,3 @@
-import pytest
 from thefuck.rules.systemctl import match, get_new_command
 from tests.utils import Command