[css-overflow-4] Remove request to not expose line-clamp longhands (#12400)

This was only intended as a temporary "hold your horses" warning while
we figured the syntax space, but the design has been stable for years at
this point, and the notion of unexposed longhands brings its own set of
complexity.

See #10439

Author: frivoal

css-overflow-4/Overview.bs

@@ -823,14 +823,6 @@ Limiting Visible Lines: the 'line-clamp' shorthand property</h3>
 	The 'line-clamp' property is a <a>shorthand</a>
 	for the 'max-lines', 'block-ellipsis', and 'continue' properties.
 
-	Issue: For the time being,
-	experimental implementations are encouraged
-	to follow the full behavior defined by this shorthand and its longhands,
-	but to only expose the shorthand to authors.
-	This is in order to facilitate further tweaking,
-	and in particular potential renaming,
-	of the longhand properties and their values.
-
 	It allows limiting the contents of a block container
 	to the specified number of lines;
 	remaining content is fragmented away