[no sq] Move shaders & remove dead Irrlicht tests (#15006)

* Move irrlicht shaders to correct place

* Remove unused Irrlicht tests

client/shaders/Irrlicht/Renderer2D.fsh

@@ -0,0 +1,23 @@
+#version 100
+
+precision mediump float;
+
+/* Uniforms */
+
+uniform int uTextureUsage;
+uniform sampler2D uTextureUnit;
+
+/* Varyings */
+
+varying vec2 vTextureCoord;
+varying vec4 vVertexColor;
+
+void main()
+{
+	vec4 Color = vVertexColor;
+
+	if (bool(uTextureUsage))
+		Color *= texture2D(uTextureUnit, vTextureCoord);
+
+	gl_FragColor = Color;
+}