Revert "added plain text representation"

This reverts commits 9dc3049d22c16aa3e102cc53ad6905a3ea6d4e8b and
ed680ffe45d309481cc9ad979d583abc820331cf.
restored simplified Turtle format

after repeated discussion, this was deemed the more desirable approach
by TPTB in order to avoid adding a non-RDF representation

addendum to the rationale: empirical evidence suggests that concepts are
of no interest in this context, so including them was deemed unnecessary

This reverts commit 49ec8bbbdd70d080b10508d25213dfb5c57ea357, restoring
commit e5c7465efc95a2b8ca84b29a2055514dd22a9810.

Conflicts:
	test/functional/api_test.rb