Fix SimplePolicy blocking account updates

This fixes an oversight in e99e2407f3
which added background_removal as a possible SimplePolicy setting.
However, it did _not_ add a default value to the base config and
as it turns out instance_list doesn’t handle unset options well.

In effect this caused federating instances with SimplePolicy enabled
but background_removal not explicitly configured to always trip up for
outgoing account updates in check_background_removal (and incoming
updates from Sharkey).
For added ""fun"" this error was able to block account updates made
e.g. via /api/v1/accounts/update_credentials.

Tests were unaffected since they explicitly override
all relevant config options.

Set a default to avoid all this
(note to self: don’t forget next time, baka!)

config/config.exs

@@ -377,6 +377,7 @@ config :pleroma, :mrf_simple,
   accept: [],
   avatar_removal: [],
   banner_removal: [],
+  background_removal: [],
   reject_deletes: [],
   handle_threads: true