Remove apparently obsolete code in iris.save

[pp-mo]

I suspect that [this line] does not do anything useful, but recent testing changes have to accommodate it, e.g.
Notably here, and this supporting code,
which were introduced in #3411

This is now affecting #4913, which is also having to work around testing for this usage.
But I can't see that this usage does any good (!)

UPDATE:

This fails in iris.tests.test_file_save, demonstrating that there was originally an intention to support saving to an open file (handle), which is what this code was supposed to enable.
So, at present, this does work for PP files (as tested in iris/tests/test_file_save.py), but not for netcdf saves.

Not knowing about this, I recently wrote a PoC patch to add saving to an open netCDF4.Dataset object, via a different mechanism here
TODO: possibly should fix to support this mode?

[pp-mo]

Result : it is not actually obsolete !
This corresponds to passing an open file object as the 'target' of iris.save (it is these which have a 'name' property).
Though, it seems only usable for DOT+PNG at present, which we have now agreed to remove : #4515.