test_ast.rb: Remove a needless line

`RubyVM::AST.parse_file` was fixed to raise `SyntaxError`
by r63602. So this line is needless.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63704 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_ast.rb

@@ -63,9 +63,7 @@ class TestAst < Test::Unit::TestCase
 
     def ast
       return @ast if defined?(@ast)
-      ast = RubyVM::AST.parse_file(@path)
-      raise "Syntax error: #{@path}" if ast.nil?
-      @ast = ast
+      @ast = RubyVM::AST.parse_file(@path)
     end
 
     private