[BUGFIX] Disable bulk delete on data sets

[Fixes #71387092]
https://www.pivotaltracker.com/story/show/71387092

Given the dependency between stagecraft and backdrop, bulk deletion
raises a lot of difficult problems. It has been decided that we don't
actually need bulk deletion so this change removes the feature from the
admin UI.
Merge pull request #147 from alphagov/disable-data-set-actions

[BUGFIX] Disable bulk delete on data sets