Remove redundant mandatory check

[okkez]

No description provided.

[okkez]

config_param :zabbix_server, :string means that Fluentd will check parameter zabbix_server is mandatory on boot.
So removed code in this PR is redundant.

@fujiwara Do you have any concern?

[fujiwara]

Thank you.
Certainly, Fluentd raises Fluent::ConfigError when zabbix_server is not specified.

<match ping.**>
  @type zabbix
  host "localhost"
  name_key_pattern "."
</match>

2017-09-26 15:13:22 +0900 [error]: config error file="fluentd.conf.zabbix" error_class=Fluent::ConfigError error="'zabbix_server' parameter is required"

[okkez]

Thank you!!