Remove unused billboard shader

data/shaders/billboard.frag

@@ -1,12 +0,0 @@
-uniform sampler2D tex;
-
-in vec2 uv;
-in vec4 vertex_color;
-out vec4 FragColor;
-
-void main(void)
-{
-    vec4 color = texture(tex, uv);
-    color *= vertex_color;
-    FragColor = vec4(color.a * color.rgb, color.a);
-}

data/shaders/billboard.vert

@@ -1,22 +0,0 @@
-uniform mat4 color_matrix;
-uniform vec3 Position;
-uniform vec2 Size;
-
-#ifdef Explicit_Attrib_Location_Usable
-layout(location = 0) in vec2 Corner;
-layout(location = 3) in vec2 Texcoord;
-#else
-in vec2 Corner;
-in vec2 Texcoord;
-#endif
-
-out vec2 uv;
-out vec4 vertex_color;
-
-void main(void)
-{
-    uv = Texcoord;
-    vec4 Center = ViewMatrix * vec4(Position, 1.);
-    gl_Position = ProjectionMatrix * (Center + vec4(Size * Corner, 0., 0.));
-    vertex_color = color_matrix[gl_VertexID];
-}