Remove 'remap_cube_dimensions'.

Plain ol' cube indexing seems really simple compared to the old routine.
I guess it may lose some efficiency though ?

When merged with test fixes from https://github.com/cpelley/iris/pull/15 and https://github.com/cpelley/iris/pull/14, all tests pass as they were (i.e. except the new failure in https://github.com/cpelley/iris/pull/14).