Removed mult_commute axiom from comm_semiring axclass.

--- a/src/HOL/Ring_and_Field.thy	Tue Apr 19 00:14:27 2005 +0200
+++ b/src/HOL/Ring_and_Field.thy	Tue Apr 19 10:59:31 2005 +0200
@@ -31,7 +31,6 @@
 axclass semiring_0_cancel \<subseteq> semiring_0, cancel_ab_semigroup_add
 
 axclass comm_semiring \<subseteq> ab_semigroup_add, ab_semigroup_mult  
-  mult_commute: "a * b = b * a"
   distrib: "(a + b) * c = a * c + b * c"
 
 instance comm_semiring \<subseteq> semiring