Remove unused error

Author: st0012

lib/irb/lc/error.rb

@@ -48,11 +48,6 @@ def initialize(val)
       super("No such job(#{val}).")
     end
   end
-  class CantShiftToMultiIrbMode < StandardError
-    def initialize
-      super("Can't shift to multi irb mode.")
-    end
-  end
   class CantChangeBinding < StandardError
     def initialize(val)
       super("Can't change binding to (#{val}).")

lib/irb/lc/ja/error.rb

@@ -48,11 +48,6 @@ def initialize(val)
       super("そのようなジョブ(#{val})はありません.")
     end
   end
-  class CantShiftToMultiIrbMode < StandardError
-    def initialize
-      super("multi-irb modeに移れません.")
-    end
-  end
   class CantChangeBinding < StandardError
     def initialize(val)
       super("バインディング(#{val})に変更できません.")