Drop never documented 'alpha' property from nodedef

Includes minimal support code for practical reasons.
We'll need it for a slightly different purpose next commit.

games/devtest/mods/testnodes/textures.lua

@@ -51,23 +51,16 @@ for a=1,#alphas do
 		groups = { dig_immediate = 3 },
 	})
 
-	-- Transparency set via "alpha" parameter
+	-- Transparency set via texture modifier
 	minetest.register_node("testnodes:alpha_"..alpha, {
 		description = S("Alpha Test Node (@1)", alpha),
-		-- It seems that only the liquid drawtype supports the alpha parameter
-		drawtype = "liquid",
+		drawtype = "glasslike",
 		paramtype = "light",
 		tiles = {
-			"testnodes_alpha.png",
+			"testnodes_alpha.png^[opacity:" .. alpha,
 		},
-		alpha = alpha,
+		use_texture_alpha = true,
 
-
-		liquidtype = "source",
-		liquid_range = 0,
-		liquid_viscosity = 0,
-		liquid_alternative_source = "testnodes:alpha_"..alpha,
-		liquid_alternative_flowing = "testnodes:alpha_"..alpha,
 		groups = { dig_immediate = 3 },
 	})
 end