[C++17 DIS comment 023] [support.runtime] Remove index entry for __bool_true_false_are_defined

[zygoloid]

[tkoeppe]

Agreed. <cstdbool> was moved to Annex D by means of Clark, and a stray index remained behind.

[zygoloid]

Proposed response: ACCEPTED

[tkoeppe]

Commit in master: d0b014a

[zygoloid]

Merged to c++17 as 7f0685f.