removed obsolete THIS_IS_ISABELLE_BUILD;

--- a/NEWS	Thu Mar 06 19:21:29 2008 +0100
+++ b/NEWS	Thu Mar 06 19:30:37 2008 +0100
@@ -153,6 +153,11 @@
 
 *** System ***
 
+* System: removed obsolete THIS_IS_ISABELLE_BUILD feature.  NB: the
+documented way of changing the user's settings is via
+ISABELLE_HOME_USER/etc/settings, which is a fully featured bash
+script.
+
 * Default settings: PROOFGENERAL_OPTIONS no longer impose xemacs ---
 in accordance with Proof General 3.7, which prefers GNU emacs.