Rather than trying to implement full i18n plural support, we just remove
this failed attempt from the past. The fact is that to get proper
support, we'd basically need to reimplement a Gettext-like plural
definition syntax within our API, then ask people to write down this
plural definition for their language, then to write every plural form…
all this for custom units which only them will ever see!
Moreover code investigation shows that the singular form was simply
never used, and the plural form was always used (whatever the actual
unit value displayed).
As for the "identifier", this was a text which was never shown anywhere
(except in the unit editor) and for all built-in units, as well as
default unitrc units, it was equivalent to the English plural value.
So we now just have a unique name which is the "long label" to be used
everywhere in the GUI, and abbreviation will be basically the "short
label". That's it. No useless (or worse, not actually usable because it
was not generic internationalization) values anymore!
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