Refactor: scheduler: remove dead code
    
    Several internal expression-testing functions were unused. While they might
    have had some potential future value, the abundance of similarly named
    functions made debugging difficult. Comment blocks were added to the functions
    they wrapped, which should add similar value.
    
    Also, we had an internal wrapper for pe__eval_date_expr() that was used
    only in crm_rule, so it was moved there.