yaml_to_mux: Remove unused import

The loader import is a leftover from when the yaml_to_mux_loader was
part of this plugin, which was separated and this import does not serve
any purpose.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

optional_plugins/varianter_yaml_to_mux/avocado_varianter_yaml_to_mux/__init__.py

@@ -20,7 +20,7 @@ import os
 import re
 import sys
 
-from avocado.core import tree, exit_codes, mux, varianter, loader
+from avocado.core import tree, exit_codes, mux, varianter
 from avocado.core.output import LOG_UI
 from avocado.core.plugin_interfaces import CLI, Varianter