Clean up comments for LWG-3940 `std::expected<void, E>::value()` also needs `E` to be copy constructible

[frederick-vs-ja]

LWG-3940 has been voted into the standard, so we should clean up the comments mentioning it.

Code:

STL/stl/inc/expected

Line 1426 in 15aea98

// per LWG-3940

STL/tests/libcxx/expected_results.txt

Line 158 in 15aea98

# libc++ doesn't speculatively implement LWG-3940

• Note that in expected_results.txt only the word "speculatively" is needed to be deleted, the whole comment should be kept.

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