Remove unused leftover artificial key _reflns_shell.d_res_limits

[jamesrhester]

_reflns_shell.d_res_limits is constructed from the upper and lower limits of a resolution shell. It was originally created to be a single key data name for this category, but is no longer used for that and is unlikely to have been used in data files. Will be backported to release candidate.

[vaitkus]

The data name has most likely not been used in any data files, but it is referenced in one of the recently published
ITG chapters [1]:

For example, according to the DDLm version of the coreCIF dictionary, the resolution value
extrema can be recorded using either a two-element list data item (_reflns_shell.d_res_limits)
or a pair of single-valued data items (_reflns_shell.d_res_low, _reflns_shell.d_res_high).

Not sure if this is a sufficient reason to not remove the item. I will also ask the editors if the chapter
itself could be edited/updated given that it most likely has only been published in the digital format.

[1] https://it.iucr.org/Gb/ch5o1v0001/