Remove default unlimited dimension on netCDF save

This PR recmoves the iris specific behaviour of assigning an 'UNLIMIDED' dimension to the outsermost dimension on save.  This is not a specified behaviour within CF and can lead to confusion or worse downstream for users of iris.

I think this is technically a bug but since it has an unknown knock-on effect with iris having had this behaviour for some time now, I thought it might be safer to simply target iris master.

@marqh and @rockdoc, I thought I would draw attention to you for this PR as I know you also wanted this bahaviour removed.