LFRic cubes don't merge due to name attribute containing filename

[jfrost-mo]

Describe the bug

When XIOS outputs LFRic to seperate streams, it requires there to be a name attribute on the cube. As this attribute will be different between the different files, we need to remove it during data loading.

How to reproduce

Steps to reproduce the behaviour:

1. Load some files with differing name attributes with the load_cube operator.
2. Observe the cubes don't merge.

Expected behaviour

We don't need this attribute, so we can remove it in the LFRic normalisation loading callback.

Environment

• Version: [e.g. from cset --version]
• Browser (if UI issue): [e.g. Chrome, Firefox]