Remove default for deprecated option facility

[lusitania]

According to the GELF Payload Specification the field facility is "optional, deprecated" and is supposed to be sent "as additional field instead".

This default breaks field overrides, see

• https://github.com/graylog-labs/gelf-rb/issues/57
• https://github.com/logstash-plugins/logstash-output-gelf/issues/8

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