remove pem/bsem from css-env-1 (w3c#13281)

Author: davidsgrogan

css-env-1/Overview.bs

@@ -260,14 +260,10 @@ For example, if ''text-size-adjust:auto''
 would cause text sizes to double,
 then ''env(preferred-text-scale)'' would resolve to ''2''.
 
-Note: The ''pem'' unit represents this same information;
-''1pem'' is exactly equivalent to ''calc(1em * env(preferred-text-scale))''.
-When directly sizing things, ''bsem'' is just a more convenient length to use.
-
 <div class=example>
 	This [=environment variable=] requires care to be used correctly.
 	By default, text scaling is applied automatically;
-	using ''env(preferred-text-scale)'' or ''pem''
+	using ''env(preferred-text-scale)''
 	would result in the scale being <em>double</em>-applied,
 	making text or UI elements too large.
 
@@ -279,7 +275,6 @@ When directly sizing things, ''bsem'' is just a more convenient length to use.
 		and when necessary scale non-text sizes by the scale factor as well.
 	* or set ''text-size-adjust:none'',
 		and then consistently use this [=environment variable=]
-		and/or the ''pem'' unit
 		to scale relevant text and UI to the user's preference.
 </div>