Review/remove handling of netcdf "conventions" attribute

[pp-mo]

As described in this comment thread, we have special handling for attributes called "conventions", but really only "Conventions" (capital-C) matters to netcdf. So we probably want to remove that.
Came up while working on Split cube-attrs project , but we really don't want to mix in that issue here.

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[pp-mo]

There seems no particular urgency to address this, and I don't think we will be addressing it as a separate issue.