How to disable old alarms on iPhone?

You might want to check that it is not still accidentally set ?

Set an alarm in Clock on iPhone - Apple Support

says:

"In the Clock app  on your iPhone, tap Alarms"

"To remove an alarm, swipe left on the alarm, then tap Delete".

___________________________________________________________________

Else: Maybe there is still a "sleep schedule" alarm that was set in the Health app?

How to: Change the next wake up alarm in Clock on iPhone - Apple Support

says:

"Although you don’t set the wake up alarm in Clock, you can make changes to it in Clock."

and

"You can remove the wake up alarm by deleting or turning off your sleep schedules.

Do one of the following: 

• Tap Change, tap Edit Sleep Schedule in Health, then turn off Sleep Schedule.
• Go to the Health app, then delete a sleep schedule or turn off all sleep schedules."

All the best :-)