After D134050, it makes sense to combine the RV64 ABI tests into a single file in order to make it more maintainable (i.e. not having to split tests based on the combinations of ABIs they're expected to impact). This patch deletes duplicated tests but doesn't do much further reorganisation beyond that.
I imagine the logical ordering of tests in the file and comments could be further improved in the future. My personal feeling is that it's probably not worth investing the time to try to get this "perfect", and to instead settle for this incremental step forward. But if there's reviewer interest in attempting to further iterate, I'm happy to do so.