This PR is *completely* lost on me.

Why are you **removing** metadata during load? And why in particular the `Conventions` attribute? I really don't understand this ... please explain.

Even if you did want to filter out an attribute there is another mechanism to do this, reference `_CF_ATTRS` and its use.

Plus, the current NetCDF saver ensures the the `Conventions` attribute does not appear as an attribute in the associated CF data variable of an Iris Cube, *and* clobbers any existing `Conventions` to the NetCDF file as a global attribute with `CF-1.5`.

I really don't think this is the behaviour your after ... or you've not quite explained the true purpose of this PR.