Make postrm commands optional

Per
<https://www.debian.org/doc/debian-policy/ch-relationships.html#binary-dependencies-depends-recommends-suggests-enhances-pre-depends>:

> Finally, the Depends field should be used if the depended-on package
> is needed by the postrm script to fully clean up after the package
> removal. There is no guarantee that package dependencies will be
> available when postrm is run, but the depended-on package is more
> likely to be available if the package declares a dependency
> (particularly in the case of postrm remove). The postrm script must
> gracefully skip actions that require a dependency if that dependency
> isn’t available.

debian/securedrop-export.postrm

@@ -22,8 +22,14 @@ set -e
 case "$1" in
     purge|remove|upgrade|failed-upgrade|abort-install|abort-upgrade|disappear)
 
-    update-desktop-database /usr/share/applications
-    update-mime-database /usr/share/mime
+    # There is no guarantees dependencies are available in postrm, so
+    # conditionally issue these updates.
+    if [[ -x "/usr/bin/update-desktop-database" ]]; then
+        update-desktop-database /usr/share/applications
+    fi
+    if [[ -x "/usr/bin/update-mime-database" ]]; then
+        update-mime-database /usr/share/mime
+    fi
     ;;
 
     *)