Remove jpeg check from generate_smallvariants.py

imghdr is not able to recognize some valid jpeg formats
Reference: https://bugs.python.org/issue28591

It was also deprecated in python 3.11 so just remove it
Reference: https://peps.python.org/pep-0594/#imghdr

Change-Id: Ideb9e35515e8c21ef951e44fc37bf4e5bb4335f5