add hook etc/kernel/postinst.d/30_remove-system-map to remove system.map

on kernel package upgrade;

self-document this package: during upgrade the following will be written
to stdout:

Setting up linux-image-4.19.0-5-amd64 (4.19.37-5+deb10u2) ...
/etc/kernel/postinst.d/30_remove-system-map:
removed '/boot/System.map-4.19.0-5-amd64

usr/lib/security-misc/remove-system.map

@@ -9,6 +9,6 @@ shopt -s nullglob
 for filename in /boot/System.map-*
 do
   if [ -f "${filename}" ]; then
-    rm -f "${filename}"
+    rm --verbose --force "${filename}"
   fi
 done