Remove duplicate problem 4clojure#56.

src/foreclojure/data_set.clj

@@ -619,18 +619,6 @@
                     "(= (__ [:b :a :b :a :b]) {:a 2, :b 3})"
                     "(= (__ '([1 2] [1 3] [1 3])) {[1 2] 1, [1 3] 2})"]})
 
-      (insert! :problems
-           {:_id 56
-            :title "Find Distinct Items"
-            :times-solved 0
-            :restricted ["distinct"]
-            :description "Write a function which removes the duplicates from a sequence. Order of the items must be maintained."
-            :tags ["medium" "seqs" "core-functions"]
-            :approved true
-            :tests ["(= (__ [1 2 1 3 1 2 4]) [1 2 3 4])"
-                    "(= (__ [:a :a :b :b :c :c]) [:a :b :c])"
-                    "(= (__ '([2 4] [1 2] [1 3] [1 3])) '([2 4] [1 2] [1 3]))"]})
-
       (insert! :problems
            {:_id 56
             :title "Find Distinct Items"