P1661 Remove dedicated precalculated hash lookup interface

[wg21bot]

P1661R0 Remove dedicated precalculated hash lookup interface (Tomasz Kamiński)

[tituswinters]

Forward to LWG for C++20 (removing pre-computed hash lookup methods from unordered_*).
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This would be a break if we did it later, should be prioritized.

[wg21bot]

P1661R1 Remove dedicated precalculated hash lookup interface (Tomasz Kamiński)

[wg21bot]

Adopted 2019-07.