Fix T61129 Eevee: alpha texture shadow bug

This was due to some shading resources not being bound for the shadow
shader. This commit completely remove the shading part when in a shadow
shader. Thus making it a bit more lightweight and removing the needed
sources.

source/blender/draw/engines/eevee/shaders/lit_surface_frag.glsl

@@ -164,6 +164,10 @@ void CLOSURE_NAME(
 	out_refr = vec3(0.0);
 #endif
 
+#ifdef SHADOW_SHADER
+	return;
+#endif
+
 	/* Zero length vectors cause issues, see: T51979. */
 	float len = length(N);
 	if (isnan(len)) {