record_transformer: remove_keys processing should be last

[repeatedly]

Currently, remove_keys doesn't work with renew_time_key parameter, so
user needs one more filter to remove renew_time_key field from the record.

<filter pattern>
  @type record_transformer
  renew_time_key event_time
  auto_typecast true
  <record>
    event_time ${Convert record["time_str"] into unixtime or float type}
  </record>
</filter>

<filter pattern>
  @type record_transformer
  remove_keys event_time
</filter>

This is verbose and not good performance. remove_keys should remove renew_time_key.

<filter pattern>
  @type record_transformer
  renew_time_key event_time
  remove_keys event_time
  auto_typecast true
  <record>
    event_time ${Convert record["time_str"] into unixtime or float type}
  </record>
</filter>

[repeatedly]

@sonots Could you check this patch?

[sonots]

LGTM