GB197 18.05.2 Remove concept "boolean" and use convertible_to<bool> instead

[wg21bot]

Remove concept boolean, replace that requirement with convertible_to<bool>
The concept boolean simultaneously:

• has an overly-complex specification
• is costly to check
• doesn't achieve what it was designed to do
  ​Replace Boolean with convertible_to<bool> ericniebler/stl2#389 expands on more of this problem.
  The best solution forward is to remove std::boolean and replace it with std::convertible_to<bool>.

Proposed change:
Strike [concept.boolean].
Replace all occurrences of boolean in the CD with convertible_to. Known occurrences:
[cmp.concept]
[concept.equalitycomparable]
[concept.totallyordered]
[concept.predicate]

[wg21bot]

Duplicate of US195 #193.