Remove frontend 'DEFAULT_CORPUS_ATTRS' mention

'DEFAULT_CORPUS_ATTRS' is not actually used in the frontend anymore, so this mention is useless / confusing now.

changed milestone to %Arkindex 1.6.1

requested review from @babadie

assigned to @mlbonhomme

I see it's still exported, then used in addDefaultCorpus, but that is not called anywhere (except a unit test).

It will be removed in frontend#1140 (closed)

merged