Remove importing of conditions from the product/conditions directory.

It's not currently in use since all the yaml files are disabled.

YAMLImportExportMixin is responsible for importing conditions and only calls
Condition#export_to_array or Condition#import_from_hash.

All of the other stuff is unused.
sync_from_file is now unused.
These were only used by the now deleted product/conditions yamls.
from_file is now unused, delete it.
Remove unused methods
from_yml and from_xml are now unused, delete them.
Seed is now unused, goodbye.
ConditionXmlHandler is now unused, :scissors:
It's 6 years later and later never happened.
Remove commented code.