Remove --disable-gems in assert_in_out_err

assert_in_out_err adds --disable=gems so we don't need to add
--disable-gems in the args list.

Author: peterzhu2118

tests/json_generator_test.rb

@@ -233,7 +233,7 @@ def test_buffer_initial_length
 
   def test_gc
     if respond_to?(:assert_in_out_err) && !(RUBY_PLATFORM =~ /java/)
-      assert_in_out_err(%w[-rjson --disable-gems], <<-EOS, [], [])
+      assert_in_out_err(%w[-rjson], <<-EOS, [], [])
         bignum_too_long_to_embed_as_string = 1234567890123456789012345
         expect = bignum_too_long_to_embed_as_string.to_s
         GC.stress = true