Remove deprecated lib/iris/fileformats/ff.py

[lbdreyer]

The module lib/iris/fileformats/ff was deprecated in Iris 1.10

https://github.com/SciTools/iris/blob/master/lib/iris/fileformats/ff.py#L20

[lbdreyer]

This includes removing:

• iris.fileformats._ff.Grid
• iris.fileformats._ff.ArakawaC
• iris.fileformats._ff.NewDynamics
• iris.fileformats._ff.ENDGame
• iris.fileformats._ff.FFHeader
• iris.fileformats._ff.FF2PP
• iris.fileformats._ff.load_cubes
• iris.fileformats._ff.load_cubes_32bit_ieee

which are imported via iris.fileformats.ff

[DPeterK]

@lbdreyer actually I think we do not want to remove the above items from _ff.py as they're all still being used – see, for example, _ff_replacement.py L29, which uses FF2PP. While all of this functionality is deprecated, it's also private so no-one should be using it directly (and it won't emit deprecation warnings either).

We can't remove this yet as there's no replacement for the functionality! But the replacement may be mule in the future, at which point we can remove all of this. As such I'm going to tighten the scope of this issue to just the removal of ff.py – the deprecated public interface.

BTW, this is all a write-up of an offline conversation with @pp-mo, who originally put the ff deprecations in place.