This is not mission editing: it is changing the game.
Essentially it can't be done. If you use the forum search function and hunt persistently you will find various workaround proposals. I doubt any of them are very satisfactory. This view exists to provide some compensation for the lack of peripheral vision, so don't do away with it lightly.
I suggest that you design your mission in such a way that either it discourages the player from using 3rd person view, or it doesn't matter if he does.