Remove unneeded flat out

We can share the same uniform name, esp flat out is not working
for my adreno 306 in android 5.0

data/shaders/simple_particle.frag

@@ -1,16 +1,16 @@
 uniform sampler2D tex;
 uniform sampler2D dtex;
+uniform float billboard;
 
 in vec2 tc;
 in vec4 pc;
-flat in float billboard_mix;
 out vec4 FragColor;
 
 #stk_include "utils/getPosFromUVDepth.frag"
 
 void main(void)
 {
-    float billboard_alpha = mix(1.0, texture(tex, tc).a, billboard_mix);
+    float billboard_alpha = mix(1.0, texture(tex, tc).a, billboard);
 #if defined(Advanced_Lighting_Enabled)
     vec2 xy = gl_FragCoord.xy / u_screen;
     float FragZ = gl_FragCoord.z;

data/shaders/simple_particle.vert

@@ -23,7 +23,6 @@ in vec2 quadcorner;
 in float anglespeed;
 #endif
 
-flat out float billboard_mix;
 out vec2 tc;
 out vec4 pc;
 
@@ -34,13 +33,11 @@ void main(void)
         gl_Position = vec4(0.);
         pc = vec4(0.0);
         tc = vec2(0.0);
-        billboard_mix = 0.0;
         return;
     }
 
     float lifetime = color_lifetime.w;
     float alpha = mix(smoothstep(1.0, 0.8, lifetime), lifetime, billboard);
-    billboard_mix = billboard;
     vec4 particle_color = vec4(color_lifetime.zyx, 1.0) * alpha;
     tc = Texcoord;