Remove filerenamehelper and associated update

[andybroomfield]

I think this file: src/FieldRenameHelper.php
should be removed.

It was part of an update to rename field_* to localgov_* early in LGD. It hasn't been required for many years.
I think its strange and allows for possible destructive behaviours if this class is left in place as it's technically a supported API, even though it is internal.

Do we need to do any depreciation process for this class? Is it safe to assume it's just internal?

I would.

1. Remove the file.
2. Remove localgov_core_update_8001
3. Add localgov_core_update_last_removed() and return 8001.