Fix GCC 4.6's new -Wunused-but-set-variable warnings.

Most instances were dead code; for those, I removed the assignments.
Some were pieces of info we don't currently plan to use, but which
we might in the future.  For those, I added an explicit cast-to-void
to indicate that we know that the thing's unused.  Finally, one was
a case where we were testing the wrong variable in a unit test.
That one I fixed.

This resolves bug 3208.