In one of my old phones, when I flashed a custom ROM, the Device information screen showed the number of the device the ROM had been created from. It still worked fine with my number and no one ever "got into" the phone.
The number is there for your information only, in case you forget your number.
As for deleting accounts and numbers:
You can't delete the only Google account you have on the phone. Add a different Google account and you can delete either of them. (Android is closely tied to the Google world.) Deleting any other account - Facebook, email, etc., is just going into Accounts and deleting the account.
To delete a number, go into Contacts, to that number, Edit and change the number or delete the account.
(And please put your ICE number on your lock screen to make it easier for first responders to contact someone if you're unable to communicate. It could save your life. Even if you put something like "If found, please call xxx xxx-xxxx", they'll call that number to see if you're allergic to anything, are diabetic, etc.)