analyzer: fix comment

I rewrote class impl_region_model_context to avoid using multiple
inheritance during patch review but forgot to update this comment.

Fix it.

gcc/analyzer/ChangeLog:
	* engine.cc (class impl_region_model_context): Fix comment.

gcc/analyzer/ChangeLog

+2020-01-15  David Malcolm  <dmalcolm@redhat.com>
+
+	* engine.cc (class impl_region_model_context): Fix comment.
+
 2020-01-14  David Malcolm  <dmalcolm@redhat.com>
 
 	PR analyzer/93212

gcc/analyzer/engine.cc

@@ -64,7 +64,7 @@ along with GCC; see the file COPYING3.  If not see
 
 static int readability_comparator (const void *p1, const void *p2);
 
-/* class impl_region_model_context : public region_model_context, public log_user.  */
+/* class impl_region_model_context : public region_model_context.  */
 
 impl_region_model_context::
 impl_region_model_context (exploded_graph &eg,