Time of flight T = (2u sin⁡θ)/g 
Average velocity = (change in displacement)/time

(You may draw the figure)
From the figure, it can be said AB is horizontal. So there is no effect of vertical component of the velocity during this displacement. 
So because the body moves at a constant speed of ‘u cos θ’ in horizontal direction. 
The average velocity during this displacement will be u ‘cos θ ‘ in the horizontal direction.