Don't load module-extras.php if Jetpack is not active

If Jetpack is not active and not in development mode, we don't really need module-extras.php, otherwise some things (like Featured Content) still work despite Jetpack not being connected to WordPress.com, which may not be the desired behavior. This was broken in 5761feaae0adb66fca66cc64080553d60b3852b7.