gl-2.0: remove unnecessary assert
Current optimisations in NIR based Mesa drivers can end up removing the entire if from the code below when they detect gl_SecondaryColor is undefined.
if (draw_secondary)
gl_FragColor = gl_SecondaryColor;
else
gl_FragColor = gl_Color;
Since the if is detected as unrequired the draw_secondary uniform will also be optimised away. So here we remove the assert().
Edited by Timothy Arceri