Fix duplicate shader version in the flat shader test

Unbreaks the flat shading test on Adreno (ended up in trying to link a
 #version 300 and a #version 320 shader together which it didn't like)

UI/GPUDriverTestScreen.cpp

@@ -183,7 +183,6 @@ static const std::vector<Draw::ShaderSource> fsFlat = {
 
 static const std::vector<Draw::ShaderSource> vsFlat = {
 	{ GLSL_3xx,
-	"#version 300 es\n"
 	"in vec3 Position;\n"
 	"in vec2 TexCoord0;\n"
 	"in lowp vec4 Color0;\n"