Change the message for removing all tags from mappings

- The message reads: "Tags removed from num_x mappings".
- This is an alternative to "Tags a and b removed from num_x
  mappings", which I don't think could be done due to tag_list being
  blank at the time of saving when tags are deleted.
Merge pull request #210 from alphagov/fix_all_tags_removed_message

Change the message for removing all tags from mappings