remove nonfree part

sources/extra_files/app/preferences.d/non-free_policy

@@ -1,4 +0,0 @@
-Explanation: Disable packages from non-free tree by default (Zabbix app install with non-free option enable)
-Package: *
-Pin: release o=Debian,a=stable,l=Debian,c=non-free
-Pin-Priority: -1

sources/extra_files/app/preferences.d/snmp-mibs-downloader_nonfree

@@ -1,4 +0,0 @@
-Explanation: Enable package snmp-mibs-downloader from non-free tree (Zabbix app install with non-free option enable)
-Package: snmp-mibs-downloader
-Pin: release o=Debian,a=stable,l=Debian,c=non-free
-Pin-Priority: 600