Remove tensor representation backend

[Prof Garth Wells]

See https://groups.google.com/forum/#!topic/fenics-dev/eZy-ZtAjTBY.

[Miklós Homolya]

Just a note: TSFC has a tensor mode which imitates what the tensor representation would do.

[Prof Garth Wells]

Nice to have it combined - better than having a proliferation of backends, which leads to inconsistencies.

[Miklós Homolya]

TSFC modes share the same UFL -> GEM and GEM -> C pipelines, but they implement different GEM -> GEM transformations on the intermediate representation.

The tensor mode, in particular, attempts to refactorise the expression such that the summation over quadrature points can be evaluated at compile time, thus the quadrature loop is eliminated.

[Jan Blechta]

Tensor representation has been removed some time ago.