Remove /etc/default/atftpd with syntax failure

Sophie Brunrequested to merge
fix-upgrade into master

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=994808 We have to modify in atftpd.prerm file because during upgrade the prerm of the installed version fails. I disable the 'set -e' before the test because the command '. /etc/default/atftpd' fails and makes the prerm fail despite its inclusion in a if statement.

Closes: #994808

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