Remove one fixed reference to height in code.

First of all, thanks for this great module!

I have a proposal for a minor addition. When using hook_physical_dimension_info_alter() to change the order of the dimensions and/or rename one or more dimensions names, things get mixed up in the widget form. Currently there is the underlying assumption, that the height field is the last one and should therefore be suffixed with the dimension unit if the unit itself is not selectable. It would be better to drop that assumption and to simply add the suffix to the last field. See the patch.