84607: Hide (not just disable) spellchecker pref UI, if there is no spellchecker installed.

r=ducarroz, sr=bienvenu, a=asa

mailnews/compose/prefs/resources/content/pref-composing_messages.xul

@@ -44,7 +44,7 @@
 
       if (!spellChecker) {
         var spellCheckerCheckbox = document.getElementById("spellCheckBeforeSend");
-        spellCheckerCheckbox.setAttribute("disabled","true");
+        spellCheckerCheckbox.setAttribute("hidden","true");
       }
     }
   ]]>