filter_by_identity fails when identities returns an itertools.chain object #170

At versions 1.8.9.0 to 1.9.0.1, filter_by_identity fails when identities returns an itertools.chain object

cfdm/constructs.py",
line 1980, in _filter_by_identity
     generator.close()
AttributeError: 'itertools.chain' object has no attribute 'close'

In this case, the itertools.chain object is being passed in by cf-python (https://github.com/NCAS-CMS/cf-python/blob/v3.11.0/cf/mixin/propertiesdata.py#L4808).

I don't think that the generator needs closing here - I can't divine any memory or speed benefit, so I think just deleting that line will suffice. PR to follow.