refactor(mail): send the queue only from the scheduler
Kernel::terminate() no longer flushes the mail queue after the response, and the USE_CRON constant is removed with it: mail:send-pending is registered as a scheduled task and runs every minute, so the queue has one owner. The baseline entry for the always-true negation of USE_CRON is dropped too.
Also merges the two consecutive isWebRuntime() blocks in handle() into one.
Also merges the two consecutive isWebRuntime() blocks in handle() into one.