Prevent duplicate values for CODE_* fields being logged to journal.

As code location fields CODE_FILE, CODE_LINE, and CODE_FUNC are
handled by libreport, we do not want the automatic values supplied by
the sd_journal API.

Without suppressing these, both the actual log event source and the
call to sd_journal_send() are logged. For example:

	[...]
        "CODE_FILE" : [ "logging.c", "abrtd.c" ],
        "CODE_LINE" : [ "143", "629" ],
        "CODE_FUNC" : [ "log_handler", "main" ],
        "MESSAGE" : "Init complete, entering main loop",
	[...]
Merge pull request #302 from twaugh/duplicate-location

Prevent duplicate values for CODE_* fields being logged to journal.