Remove unneeded marker definitions from pytest.ini

Those are now defined in pytest core or pytest-rerunfailures

pytest.ini

@@ -13,11 +13,9 @@ markers =
     frozen: Tests which can only be run if sys.frozen is True.
     integration: Tests which test a bigger portion of code
     end2end: End to end tests which run qutebrowser as subprocess
-    skip: Always skipped test.
     pyqt531_or_newer: Needs PyQt 5.3.1 or newer.
     xfail_norun: xfail the test with out running it
     ci: Tests which should only run on CI.
-    flaky: Tests which are flaky and should be rerun
 qt_log_level_fail = WARNING
 qt_log_ignore =
     ^SpellCheck: .*