Clear APPSIGNAL_FILTER_PARAMETERS in filter_test.exs
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.

appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir