Revert "Fix casting a DateTime to an array"

Summary: This reverts commit 728b3674e2a89ee5404031e821e96b906d0b945e.

D1414803 causes
flib/intern/site/web/recruiting/stat/__tests__/SchedulrStatEngineTest.php to
fail

Reviewed By: @fredemmott

Differential Revision: D1451208
typechecker: don't assert requirement is not a trait

Summary: ... when we switched error handling to allowing multiple
 errors, we allowed evaluation to continue after naming threw an error
 for 'extends/implements' of constructs of incorrect type. We should
 probably fix the nast check.

Reviewed By: @gabelevi

Differential Revision: D1451660