You'll need a registry editor such as the PHM Registry Editor, which is a freeware.
Disable the SMS Sent notification
Create the following key in your phone registry:
HKLM\Software\Microsoft\Inbox\Settings\SMSNoSentMsg
Value = 1 (DWORD decimal)
Note that the 'Settings' key may not exist in your device, and thus may have to be created first.
(Arghh! It keeps adding a space in Msg - obviously, it should be one word)