formatter: remove unnecessary codes

Co-authored-by: Daijiro Fukuda <fukuda@clear-code.com>

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

lib/fluent/compat/formatter.rb

@@ -104,15 +104,6 @@ class CsvFormatter < Fluent::Plugin::CsvFormatter
 
         # Do not cache because it is hard to consider the thread key correctly.
         # (We can try, but it would be low priority.)
-        def csv_for_thread
-          CSV.new("".force_encoding(Encoding::ASCII_8BIT), **@generate_opts)
-        end
-
-        # It is hard to consider the thread key correctly for the compat layer.
-        def csv_thread_key
-          raise NotImplementedError, "Compat CsvFormatter does not support CSV cache. Do not use this method."
-        end
-
         def csv_cacheable?
           false
         end