Define a drop_diversion function in the postrm script

For consistency with other kali-* packages that use a similar diversion
machinery.

debian/kali-themes.postrm

@@ -33,13 +33,18 @@ FILES_TO_DIVERT="
 /etc/xdg/yakuakerc
 " # END FILES_TO_DIVERT
 
+drop_diversion() {
+    local file="$1"
+    dpkg-divert --rename --package kali-themes \
+		--divert "$file.original" \
+		--remove "$file"
+}
+
 case "$1" in
     remove)
 	for file in $FILES_TO_DIVERT; do
 	    rm -f "$file"
-	    dpkg-divert --rename --package kali-themes \
-	            --divert "$file.original" \
-		    --remove "$file"
+	    drop_diversion "$file"
 	done
 	# Recover GNOME backgrounds
 	file=/usr/share/gnome-background-properties/gnome-backgrounds.xml