[ruby/openssl] ssl: remove private method SSLSocket#tmp_ecdh_callback

Commit ee037e146037 ("ssl: remove SSL::SSLContext#tmp_ecdh_callback",
2020-08-12) forgot to remove the method.

https://github.com/ruby/openssl/commit/bef9ea84e4

ext/openssl/lib/openssl/ssl.rb

@@ -432,10 +432,6 @@ YoaOffgTf5qxiwkjnlVZQc3whgnEt9FpVMvQ9eknyeGB5KHfayAc3+hUAvI3/Cr3
         @context.tmp_dh_callback || OpenSSL::SSL::SSLContext::DEFAULT_TMP_DH_CALLBACK
       end
 
-      def tmp_ecdh_callback
-        @context.tmp_ecdh_callback
-      end
-
       def session_new_cb
         @context.session_new_cb
       end