GELF appender: don't send full_message field

According to the GELF spec v1.1 [1], the full\_message field in GELF is optional. The log4js implementation until now has sent identical short\_message and full\_message fields. Since this does not add any new information to the log message and will use double the necessary storage space, I suggest that full\_message be dropped from the GELF appender.

[1] http://graylog2.org/gelf#specs