Retire the 'middle_face' field in 'struct grid', together with the

Retire the 'middle_face' field in 'struct grid', together with the
overly complicated algorithm that uses it to home in on the grid edge
closest to a mouse click. That algorithm is being stressed beyond its
limit by the new grid type, and it's unnecessary anyway given that no
sensibly sized puzzle grid is going to be big enough to make it
prohibitively expensive just to do the trivial approach of iterating
over all edges and finding the closest of the eligible ones.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9108 cda61777-01e9-0310-a592-d414129be87e