Fix yunohost-config-dspam postrm

brackets are not sh-compatible

admin@yunohost:~$ sh -c "ls /etc/dspam/{dspam.conf,default.prefs}"
ls: cannot access /etc/dspam/{dspam.conf,default.prefs}: No such file or directory

admin@yunohost:~$ bash -c "ls /etc/dspam/{dspam.conf,default.prefs}"
/etc/dspam/default.prefs  /etc/dspam/dspam.conf