Remove incorrect comment about core_hint_black_box

[divergentdave]

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This removes a comment from Cargo.toml on the core_hint_black_box feature. Since #123 partially undid #107, the core_hint_black_box feature is still used.

FWIW, #107 also removed some documentation about the feature, but I figure that probably needs more wordsmithing before it is reintroduced, based on recent discussions.

[tarcieri]

Yeah, we should definitely add some more explanatory comments