Remove the include_children option from /with_tags

This was used in an early version of the browse pages (when they were single pages that were JS driven).  It's no longer used anywhere, so best removed to reduce complexity.
Inline the modifiers array as it's now a single entry
Remove redundant comment.
Merge pull request #98 from alphagov/remove_include_children

Remove the include_children option from /with_tags