test_string.rb: remove duplicated code

* test/ruby/test_string.rb (TestString#test_frozen_dstring): remove
  duplicated assertion.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42778 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_string.rb

@@ -2223,13 +2223,7 @@ class TestString < Test::Unit::TestCase
   end
 
   def test_frozen_string_cannot_be_adjacent
-    assert_raises(SyntaxError) do
-      eval(%{
-        "hello"f "world"
-      })
-    end
-
-    assert_raises(SyntaxError) do
+    assert_raise(SyntaxError) do
       eval(%{
         "hello"f "world"
       })