reduce dependency (toward move to 'HOL')

--- a/src/HOL/Library/Infinite_Set.thy	Thu Nov 28 13:58:11 2013 +0100
+++ b/src/HOL/Library/Infinite_Set.thy	Thu Nov 28 13:58:12 2013 +0100
@@ -5,7 +5,7 @@
 header {* Infinite Sets and Related Concepts *}
 
 theory Infinite_Set
-imports Main
+imports Set_Interval
 begin
 
 subsection "Infinite Sets"
@@ -554,4 +554,3 @@
   by (simp add: atmost_one_def)
 
 end
-