[css-text-3] Drop reference to HTTP Content-Language per #5031

Author: fantasai

css-text-3/Overview.bs

@@ -185,9 +185,8 @@ Languages and Typesetting</h3>
     or content in a <a>document language</a> that does not have a language-tagging facility
     is considered to have an unknown <a>content language</a>.
 
-    Note: Authors can tag content using the global <code>lang</code> attribute in HTML,
-    the universal <code>xml:lang</code> attribute in XML,
-    and the HTTP <code>Content-Language</code> header for content served over HTTP.
+    Note: Authors should tag content using the global <code>lang</code> attribute in HTML,
+    the universal <code>xml:lang</code> attribute in XML.
 
     The [=content language=] an element is declared to be in
     also identifies the specific written form of that language used in that element,