Bug 894283 - [Clock] Remove in-app alarm enabled icon

https://bugzilla.mozilla.org/show_bug.cgi?id=894283

- Per accepted proposal: https://bugzilla.mozilla.org/show_bug.cgi?id=894283#c10
- Removes #clock-hour24-state.alarm-set-indicator::after CSS rule
- Removes ClockView.showHideAlarmSetIndicator

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>