First of all, oyster shell is not grit as it's water soluble. It should be offered on the side so hens can eat what each needs for their own calcium levels. Mixing it into feed deprives them of this ability to self regulate.
Corn is not a nutritious food, having mostly empty calories, sort of like you eating nothing but corn or potato crisps for three meals a day. (You don't do that, do you?)
Greens don't have the levels of protein and other nutrients hens needs for building eggs.
Unless you feed a well balanced commercial layer or all flock feed, your hens will eventually stop laying eggs due to malnutrition.
Corn is 8 - 9% protein. It is also lacking in other major nutrients. A blended layer feed is 16% protein. And it has been amended to have the right amino acids in the right proportions, as well as the necessary vitamins and minerals. If you feed a diet of corn, in addition to ceasing to lay, as aart says, you may very well end up with a flock of very ill birds who resort to cannibalism to supplement their deficient diet.