Removed _diffrn.id names that belong in multi-block dictionary.

[jamesrhester]

Going through core CIF as I revise the core CIF chapter in Vol G, I found that _diffrn.id and friends are in core while also being in multiblock. This PR removes them.

This also means removing _cell.diffrn_id. My understanding of #442 is that this data name was a mistake and was probably never used. If necessary, we can add it as a deprecated data name in the multi-block dictionary.

Once merged I will backport this to rc-3.3.0.

[vaitkus]

The cell-measurement-multi-block.cif file in the examples/ directory should most likely also be moved to cif_core_multiblock repository as it uses these data names. Alternatively, it should be updated to explicitly state that it uses the multiblock dictionary.

I think it somewhat makes sense to at least leave a copy of this example file in this repository since together with cell-measurement-single-block.cif they showcase the old and the new (recommended) way of organizing data side by side.