Clear APPSIGNAL_FILTER_PARAMETERS in filter_test.exs

Since 4fb068, APPSIGNAL_FILTER_PARAMETERS gets set from
Appsignal.write_to_environment/1. This commit clears it in
filter_test.exs, so it doesn't get set into the application config in
the next test.

test/transaction/filter_test.exs

@@ -70,6 +70,7 @@ defmodule AppsignalTransactionFilterTest do
     Application.put_env(app, key, value)
     function.()
     Application.delete_env(app, key)
+    System.delete_env("APPSIGNAL_FILTER_PARAMETERS")
   end
 
 end