Revert "test: use binary mode explicitly for Ruby 2.7"

This reverts commit 736174d.

It doesn't solve anything.

Author: kou

test/csv/test_encodings.rb

@@ -83,7 +83,7 @@ def test_read_with_default_encoding
         no_warnings do
           Encoding.default_external = encoding
         end
-        result = CSV.read(@temp_csv_path, "rb")[0][0]
+        result = CSV.read(@temp_csv_path)[0][0]
       ensure
         no_warnings do
           Encoding.default_external = default_external