Remove linebreaks filter when rendering flatpages

Line breaks are handled by TinyMCE just fine, so it can be safely
removed. Mentioned here:
https://groups.google.com/d/msgid/django-oscar/694e60bf-d3cc-42f2-a5b6-8717fdccd68b%40googlegroups.com?utm_medium=email&utm_source=footer

The filter was added without a comment two years ago in
231e1b87ec0ccca49bd622699d6199fbd02470c8, probably to get the linebreaks
in the fixture working.