[css-color] Finally remove color-mod() so that people will stop referring to it; when we add it back it will have different syntax.

Author: tabatkins

css-color-4/Overview.bs

@@ -127,7 +127,7 @@ Representing sRGB Colors: the <<color>> type</h2>
 
 	<pre class='prod'>
 	<dfn>&lt;color></dfn> = <<rgb()>> | <<rgba()>> | <<hsl()>> | <<hsla()>> |
-	          <<hwb()>> | <<gray()>> | <<device-cmyk()>> | <<color-mod()>> |
+	          <<hwb()>> | <<gray()>> | <<device-cmyk()>> |
 	          <<hex-color>> | <<named-color>> | currentcolor |
 	          <<deprecated-system-color>>
 	</pre>
@@ -239,7 +239,6 @@ Resolving Color values</h2>
 		* ''lab()'' and ''lch()''
 		* ''gray()''
 		* ''color()''
-		* ''color-mod()''
 	</div>
 
 	Issue: Define if changing the working color space should have any impact on the above.
@@ -2278,7 +2277,7 @@ Converting Between Uncalibrated CMYK and RGB-Based Colors</h3>
 		<li>fallback color must be set to the input color
 	</ul>
 
-
+<!--
 <h2 id='modifying-colors'>
 Modifying Colors: the ''color-mod()'' function</h2>
 
@@ -2777,7 +2776,7 @@ Issue: The code below is only for sRGB, and duplicates the
 
 	Note: The contrast ratio of two colors is contained within the range [1,21],
 	where two identical colors are 1 and the ratio of white and black is 21.
-
+-->
 
 <h2 id="transparency">
 Transparency: the 'opacity' property</h2>